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Move labs into 6.828 repo. The lab text isn't dependent on specific 

xv6 code. Lab submission instructions etc. are likely going to be more
MIT 6.828 specific.
feat/start
Frans Kaashoek 2019-08-20 20:23:18 -04:00
parent c612d452fd
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<html>
<head>
<title>Lab: Copy-on-Write Fork for xv6</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: Copy-on-Write Fork for xv6</h2>
<p>
Your task is implement copy-on-write fork in the xv6 kernel. You are
done if your modified kernel executes both the cow and usertests
programs successfully.
<h2>The problem</h2>
The fork() system call in xv6 copies all of the parent process's
user-space memory into the child. If the parent is large, copying can
take a long time. In addition, the copies often waste memory; in many
cases neither the parent nor the child modifies a page, so that in
principle they could share the same physical memory. The inefficiency
is particularly clear if the child calls exec(), since then most of
the copied pages are thrown away without ever being used. Of course,
sometimes both child and parent modify memory at the same virtual
address after a fork(), so for some pages the copying is truly needed.
<h2>The solution</h2>
The goal of copy-on-write (COW) fork() is to defer allocating and
copying physical memory pages for the child until they are actually
needed, in the hope that they may never be needed.
<p>
COW fork() creates just a pagetable for the child, with PTEs for user
memory pointing to the parent's physical pages. COW fork() marks all
the user PTEs in both parent and child as read-only. When either
process tries to write one of these COW pages, the CPU will force a
page fault. The kernel page-fault handler detects this case, allocates
a page of physical memory for the faulting process, copies the
original page into the new page, and modifies the relevant PTE in the
faulting process to refer to the new page, this time with the PTE
marked writeable. When the page fault handler returns, the user
process will be able to write its copy of the page.
<p>
COW fork() makes freeing of the physical pages that implement user
memory a little trickier. A given physical page may be referred to by
multiple processes' page tables, and should be freed when the last
reference disappears.
<h2>The cow test program</h2>
To help you test your implementation, we've provided an xv6 program
called cow (source in user/cow.c). cow runs various tests, but
even the first will fail on unmodified xv6. Thus, initially, you
will see:
<pre>
$ cow
simple: fork() failed
$
</pre>
The "simple" test allocates more than half of available physical
memory, and then fork()s. The fork fails because there is not enough
free physical memory to give the child a complete copy of the parent.
<p>
When you are done, your kernel should be able to run both cow and
usertests. That is:
<pre>
$ cow
simple: ok
simple: ok
three: zombie!
ok
three: zombie!
ok
three: zombie!
ok
file: ok
ALL COW TESTS PASSED
$ usertests
...
ALL TESTS PASSED
$
</pre>
<h2>Hints</h2>
Here's one reasonable plan of attack. Modify uvmcopy() to map the
parent's physical pages into the child, instead of allocating new
pages, and clear PTE_W in the PTEs of both child and parent.
Modify usertrap() to recognize a page fault. When a page fault occurs
on a COW page, allocate a new page with kalloc(), copy the old page to
the new page, and install the new page in the PTE with PTE_W set.
Next, ensure that each physical page is freed when the last PTE
reference to it goes away (but not before!), perhaps by implementing
reference counts in kalloc.c. Finally, modify copyout() to use the
same scheme as page faults when it encounters a COW page.
<p>
It may be useful to have a way to record, for each PTE, whether it is
a COW mapping. You can use the RSW (reserved for software) bits in
the RISC-V PTE for this.
</body>
</html>

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<html>
<head>
<title>Lab: file system</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: file system</h1>
<p>In this lab you will add large files and <tt>mmap</tt> to the xv6 file system.
<h2>Large files</h2>
<p>In this assignment you'll increase the maximum size of an xv6
file. Currently xv6 files are limited to 268 blocks, or 268*BSIZE
bytes (BSIZE is 1024 in xv6). This limit comes from the fact that an
xv6 inode contains 12 "direct" block numbers and one "singly-indirect"
block number, which refers to a block that holds up to 256 more block
numbers, for a total of 12+256=268. You'll change the xv6 file system
code to support a "doubly-indirect" block in each inode, containing
256 addresses of singly-indirect blocks, each of which can contain up
to 256 addresses of data blocks. The result will be that a file will
be able to consist of up to 256*256+256+11 blocks (11 instead of 12,
because we will sacrifice one of the direct block numbers for the
double-indirect block).
<h3>Preliminaries</h3>
<p>Modify your Makefile's <tt>CPUS</tt> definition so that it reads:
<pre>
CPUS := 1
</pre>
<b>XXX doesn't seem to speedup things</b>
<p>Add
<pre>
QEMUEXTRA = -snapshot
</pre>
right before
<tt>QEMUOPTS</tt>
<p>
The above two steps speed up qemu tremendously when xv6
creates large files.
<p><tt>mkfs</tt> initializes the file system to have fewer
than 1000 free data blocks, too few to show off the changes
you'll make. Modify <tt>param.h</tt> to
set <tt>FSSIZE</tt> to:
<pre>
#define FSSIZE 20000 // size of file system in blocks
</pre>
<p>Download <a href="big.c">big.c</a> into your xv6 directory,
add it to the UPROGS list, start up xv6, and run <tt>big</tt>.
It creates as big a file as xv6 will let
it, and reports the resulting size. It should say 140 sectors.
<h3>What to Look At</h3>
The format of an on-disk inode is defined by <tt>struct dinode</tt>
in <tt>fs.h</tt>. You're particularly interested in <tt>NDIRECT</tt>,
<tt>NINDIRECT</tt>, <tt>MAXFILE</tt>, and the <tt>addrs[]</tt> element
of <tt>struct dinode</tt>. Look Figure 7.3 in the xv6 text for a
diagram of the standard xv6 inode.
<p>
The code that finds a file's data on disk is in <tt>bmap()</tt>
in <tt>fs.c</tt>. Have a look at it and make sure you understand
what it's doing. <tt>bmap()</tt> is called both when reading and
writing a file. When writing, <tt>bmap()</tt> allocates new
blocks as needed to hold file content, as well as allocating
an indirect block if needed to hold block addresses.
<p>
<tt>bmap()</tt> deals with two kinds of block numbers. The <tt>bn</tt>
argument is a "logical block" -- a block number relative to the start
of the file. The block numbers in <tt>ip->addrs[]</tt>, and the
argument to <tt>bread()</tt>, are disk block numbers.
You can view <tt>bmap()</tt> as mapping a file's logical
block numbers into disk block numbers.
<h3>Your Job</h3>
Modify <tt>bmap()</tt> so that it implements a doubly-indirect
block, in addition to direct blocks and a singly-indirect block.
You'll have to have only 11 direct blocks, rather than 12,
to make room for your new doubly-indirect block; you're
not allowed to change the size of an on-disk inode.
The first 11 elements of <tt>ip->addrs[]</tt> should be
direct blocks; the 12th should be a singly-indirect block
(just like the current one); the 13th should be your new
doubly-indirect block.
<p>
You don't have to modify xv6 to handle deletion of files with
doubly-indirect blocks.
<p>
If all goes well, <tt>big</tt> will now report that it
can write sectors. It will take <tt>big</tt> minutes
to finish.
<b>XXX this runs for a while!</b>
<h3>Hints</h3>
<p>
Make sure you understand <tt>bmap()</tt>. Write out a diagram of the
relationships between <tt>ip->addrs[]</tt>, the indirect block, the
doubly-indirect block and the singly-indirect blocks it points to, and
data blocks. Make sure you understand why adding a doubly-indirect
block increases the maximum file size by 256*256 blocks (really -1),
since you have to decrease the number of direct blocks by one).
<p>
Think about how you'll index the doubly-indirect block, and
the indirect blocks it points to, with the logical block
number.
<p>If you change the definition of <tt>NDIRECT</tt>, you'll
probably have to change the size of <tt>addrs[]</tt>
in <tt>struct inode</tt> in <tt>file.h</tt>. Make sure that
<tt>struct inode</tt> and <tt>struct dinode</tt> have the
same number of elements in their <tt>addrs[]</tt> arrays.
<p>If you change the definition of <tt>NDIRECT</tt>, make sure to create a
new <tt>fs.img</tt>, since <tt>mkfs</tt> uses <tt>NDIRECT</tt> too to build the
initial file systems. If you delete <tt>fs.img</tt>, <tt>make</tt> on Unix (not
xv6) will build a new one for you.
<p>If your file system gets into a bad state, perhaps by crashing,
delete <tt>fs.img</tt> (do this from Unix, not xv6). <tt>make</tt> will build a
new clean file system image for you.
<p>Don't forget to <tt>brelse()</tt> each block that you
<tt>bread()</tt>.
<p>You should allocate indirect blocks and doubly-indirect
blocks only as needed, like the original <tt>bmap()</tt>.
<p>Optional challenge: support triple-indirect blocks.
<h2>Writing with a Log</h2>
Insert a print statement in bwrite (in bio.c) so that you get a
print every time a block is written to disk:
<pre>
printf("bwrite block %d\n", b->blockno);
</pre>
Build and boot a new kernel and run this:
<pre>
$ rm README
</pre>
<p>You should see a sequence of bwrite prints after the <tt>rm</tt>.</p>
<div class="question">
<ol>
<li>Annotate the bwrite lines with the kind of information that is
being written to the disk (e.g., "README's inode", "allocation
bitmap"). If the log is being written, note both that the log is being
written and also what kind of information is being written to the log.
<li>Mark with an arrow the first point at which, if a
crash occured, README would be missing after a reboot
(after the call to <tt>recover_from_log()</tt>).
</ol>
</p>
</div>
<h2>Crash safety</h2>
<p>This assignment explores the xv6 log in two parts.
First, you'll artificially create a crash which illustrates
why logging is needed. Second, you'll remove one
inefficiency in the xv6 logging system.
<p>
Submit your solution before the beginning of the next lecture
to <a href="https://6828.scripts.mit.edu/2018/handin.py/">the submission
web site</a>.
<h3>Creating a Problem</h3>
<p>
The point of the xv6 log is to cause all the disk updates of a
filesystem operation to be atomic with respect to crashes.
For example, file creation involves both adding a new entry
to a directory and marking the new file's inode as in-use.
A crash that happened after one but before the other would
leave the file system in an incorrect state after a reboot,
if there were no log.
<p>
The following steps will break the logging code in a way that
leaves a file partially created.
<p>
First, replace <tt>commit()</tt> in <tt>log.c</tt> with
this code:
<pre>
#include "kernel/proc.h"
void
commit(void)
{
int pid = myproc()->pid;
if (log.lh.n > 0) {
write_log();
write_head();
if(pid > 1) // AAA
log.lh.block[0] = 0; // BBB
install_trans();
if(pid > 1) // AAA
panic("commit mimicking crash"); // CCC
log.lh.n = 0;
write_head();
}
}
</pre>
<p>
The BBB line causes the first block in the log to be written to
block zero, rather than wherever it should be written. During file
creation, the first block in the log is the new file's inode updated
to have non-zero <tt>type</tt>.
Line BBB causes the block
with the updated inode to be written to block 0 (whence
it will never be read), leaving the on-disk inode still marked
unallocated. The CCC line forces a crash.
The AAA lines suppress this buggy behavior for <tt>init</tt>,
which creates files before the shell starts.
<p>
Second, replace <tt>recover_from_log()</tt> in <tt>log.c</tt>
with this code:
<pre>
static void
recover_from_log(void)
{
read_head();
printf("recovery: n=%d but ignoring\n", log.lh.n);
// install_trans();
log.lh.n = 0;
// write_head();
}
</pre>
<p>
This modification suppresses log recovery (which would repair
the damage caused by your change to <tt>commit()</tt>).
<p>
Finally, remove the <tt>-snapshot</tt> option from the definition
of <tt>QEMUEXTRA</tt> in your Makefile so that the disk image will see the
changes.
<p>
Now remove <tt>fs.img</tt> and run xv6:
<pre>
% rm fs.img ; make qemu
</pre>
<p>
Tell the xv6 shell to create a file:
<pre>
$ echo hi > a
</pre>
<p>
You should see the panic from <tt>commit()</tt>. So far
it is as if a crash occurred in a non-logging system in the middle
of creating a file.
<p>
Now re-start xv6, keeping the same <tt>fs.img</tt>:
<pre>
% make qemu
</pre>
<p>
And look at file <tt>a</tt>:
<pre>
$ cat a
</pre>
<p>
You should see <tt>panic: ilock: no type</tt>. Make sure you understand what happened.
Which of the file creation's modifications were written to the disk
before the crash, and which were not?
<h3>Solving the Problem</h3>
Now fix <tt>recover_from_log()</tt>:
<pre>
static void
recover_from_log(void)
{
read_head();
cprintf("recovery: n=%d\n", log.lh.n);
install_trans();
log.lh.n = 0;
write_head();
}
</pre>
<p>
Run xv6 (keeping the same <tt>fs.img</tt>) and read <tt>a</tt> again:
<pre>
$ cat a
</pre>
<p>
This time there should be no crash. Make sure you understand why
the file system now works.
<p>
Why was the file empty, even though you created
it with <tt>echo&nbsp;hi&nbsp;>&nbsp;a</tt>?
<p>
Now remove your modifications to <tt>commit()</tt>
(the if's and the AAA and BBB lines), so that logging works again,
and remove <tt>fs.img</tt>.
<h3>Streamlining Commit</h3>
<p>
Suppose the file system code wants to update an inode in block 33.
The file system code will call <tt>bp=bread(block 33)</tt> and update the
buffer data. <tt>write_log()</tt> in <tt>commit()</tt>
will copy the data to a block in the log on disk, for example block 3.
A bit later in <tt>commit</tt>, <tt>install_trans()</tt> reads
block 3 from the log (containing block 33), copies its contents into the in-memory
buffer for block 33, and then writes that buffer to block 33 on the disk.
<p>
However, in <tt>install_trans()</tt>, it turns out that the modified
block 33 is guaranteed to be still in the buffer cache, where the
file system code left it. Make sure you understand why it would be a
mistake for the buffer cache to evict block 33 from the buffer cache
before the commit.
<p>
Since the modified block 33 is guaranteed to already be in the buffer
cache, there's no need for <tt>install_trans()</tt> to read block
33 from the log. Your job: modify <tt>log.c</tt> so that, when
<tt>install_trans()</tt> is called from <tt>commit()</tt>,
<tt>install_trans()</tt> does not perform the needless read from the log.
<p>To test your changes, create a file in xv6, restart, and make sure
the file is still there.
<b>XXX Does this speedup bigfile?</b>
<b>XXX Maybe support lseek and modify shell to append to a file?</b>
</body>
</html>

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<html>
<head>
<title>Lab: mount/umount</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: mount/umount</h1>
<p>In this lab you will add support for mounting/unmounting of file
systems to xv6. This lab will expose you to many parts of the xv6
file system, including pathname lookup, inodes, logging/recovery, disk
driver, concurrency, etc.
<p>Your job is modify xv6 so that your modified kernel passes the
tests in mounttest. You will have to implement two system
calls: <tt>mount(char *source, char *target)</tt>
and <tt>umount(char *target)</tt>. Mount attaches the device
referenced by <tt>source</tt> (e.g., <tt>/disk1</tt>) at the
location specified by <tt>target</tt>. For
example, <tt>mount("/disk1", "/m")</tt> will attach <tt>disk1</tt>
at the directory <tt>/m</tt>. After this mount call, users can use
pathnames such as <tt>/m/README</tt> to read the
file <tt>README</tt> stored in the root directory
on <tt>disk1</tt>. <tt>Umount</tt> removes the attachment. For
example, <tt>umount("/m")</tt> unmounts disk1 from <tt>/m</tt>.
<p>There are several major challenges in implementing the mount system
calls:
<ul>
<li>Adding the actual system calls so that user programs can call
them. This is similar to previous labs in which you added
systems calls xv6.
<li>Supporting several disks. You will have generalize to
virtio_disk.c to support at least two disks.
<li>Logging file system modifications to the right disk. xv6
assumes there is only disk and file system calls typically start
with <tt>begin_op</tt> and end with <tt>end_op</tt>, logging all
modifications between these two calls to the log on the one
disk. With mount, modifications to the file system on the
second disk must be logged to the second disk.
<li>Modifying pathname lookup (<tt>namex</tt>) so that when a
lookup cross a mount point, it continues at the root inode of
the attached disk.
</ul>
<p>The rest of this assignment provides some hints how you might go
about the above challenges.
<h2>Adding system calls</h2>
<p>Add the stubs for the two systems calls to xv6 so that you can
compile mounttest and add two empty functions for the two system calls
to sysfile.c. Run mounttest and it will fail on the first call
to <tt>mount</tt>.
<h2>Adding a second disk</h2>
<p>To be able to mount another disk, you need to extend xv6 to support
at least two disks. Modify virtio_disk.c to support an array of two
disks instead of a single disk. The address of the second disk
is <tt>0x10002000</tt>; modify the macro <tt>R</tt> to take a disk
number (0, 1,..) and read/write to the memory address for that disk.
<p>All functions in <tt>virtio_disk.c</tt> need to take the disk
number as an argument to update the state of the disk that is
read/written to or to receive an interrupt from the disk.
Modify <tt>virtio_disk_init</tt> to take a disk number as an argument
and update is to that it initializes that disk. Similar, go through
the other functions; make these changes should be most mechanical
(i.e., text substitutions).
<p>The second disk interrupts at IRQ 2; modify trap.c to receive that
interrupt and <tt>virtio_disk_intr</tt> with the number of the disk
that generated the interrupt.
<p>Modify the file Makefile to tell qemu to provide a second
disk. Define the variable <tt>QEMUEXTRA = -drive
file=fs1.img,if=none,format=raw,id=x1 -device
virtio-blk-device,drive=x1,bus=virtio-mmio-bus.1</tt> and
add <tt>$(QEMUEXTRA)</tt> to the end of <tt>QEMUOPTS</tt>.
<p>Create a second disk image <tt>fs1.img</tt>. Easiest thing to do
is just copy the file <tt>fs.img</tt>. You might want to add rules
to the Makefile to make this image and remove it on <tt>make
clean</tt>.
<p>Add to the user program init a call to create a device for the new
disk. For example, add the line <tt>mknod("disk1", DISK, 1);</tt> to
init.c. This will create an inode of type device in the root
directory with major number <tt>DISK</tt> and minor number 1.
<p>The first argument of the <tt>mount</tt> system call ("disk1") will
refer to the device you created using <tt>mknod</tt> above. In your
implementation of the mount system call,
call <tt>virtio_disk_init</tt> with the minor number as the argument
to initialize the second disk. (We reserve minor number 0 for the
first disk.)
<p>Boot xv6, run mounttest, and make sure <tt>virtio_disk_init</tt>
gets called (e.g., add print statement). You won't know if your
changes are correct, but your code should compile and invoke the
driver for the second disk.
<h2>Modify the logging system</h2>
<p>After calling <tt>virtio_disk_init</tt>, you need to also
call <tt>loginit</tt> to initialize the logging system for the
second disk (and restore the second disk if a power failure happened
while modifying the second disk). Generalize the logging system to
support to two logs, one on disk 0 and one disk 1. These changes
are mostly mechanical (e.g., <tt>log.</tt> changes
to <tt>log[n].</tt>), similar to generalizing the disk driver to
support two disks.
<p>To make xv6 compile, you need to provide a disk number
to <tt>begin_op</tt> and <tt>end_op</tt>. It will be a challenge to
figure out what the right value is; for now just specify the first
disk (i.e., 0). This isn't correct, since modifications to the
second disk should be logged on the second disk, but we have no way
yet to read/write the second disk. Come back to this later when you
have a better idea how things will fit together, but make sure that
xv6 compiles and still runs.
<h2>Pathname lookup</h2>
<p>Modify <tt>namex</tt> to traverse mount points: when <tt>namex</tt>
sees an inode to which a file system is attached, it should traverse
to the root inode of that file system. Hint: modify the in-memory
inode in file.h to keep some additional state, and initialize that
state in the mount system call. Note that the inode already has a
field for disk number (i.e., <tt>dev</tt>), which is initialized and
passed to reads and writes to the driver. <tt>dev</tt> corresponds
to the minor number for disk devices.
<p>Your modified xv6 should be able to pass the first tests in
mounttest (i.e., <tt>stat</tt>). This is likely to be challenging,
however, because now your kernel will be reading from the second
disk for the first time, and you may run into many issues.
<p>Even though <tt>stat</tt> may return correctly, your code is likely
to be incorrect, because in <tt>namex</tt>
because <tt>iunlockput</tt> may modify the second disk (e.g., if
another process removes the file or directory) and those
modifications must be written to the second disk. Your job is to
fix the calls to <tt>begin_op</tt> and <tt>end_op</tt> to take the
right device. One challenge is that <tt>begin_op</tt> is called at
the beginning of a system call but then you don't know the device
that will be involved; you will have to postpone this call until you
know which inode is involved (which tells you will which device is
involved). Another challenge is that you cannot postpone
calling <tt>begin_op</tt> passed <tt>ilock</tt> because that
violates lock ordering in xv6; you should not be
calling <tt>begin_op</tt> while holding locks on inodes. (The log
system allows a few systems calls to run; if a system call that
holds an inode lock isn't admitted and one of the admitted system
calls needs that inode to complete, then xv6 will deadlock.)
<p>Once you have implemented a plan for <tt>begin_op</tt>
and <tt>end_op</tt>, see if your kernel can pass <tt>test0</tt>. It
is likely that you will have to modify your implementation of the
mount system call to handle several corner cases. See the tests
in <tt>test0</tt>.
<p>Run usertests to see if you didn't break anything else. Since you
modified <tt>namex</tt> and <tt>begin/end_op</tt>, which are at the
core of the xv6 file system, you might have introduced bugs, perhaps
including deadlocks. Deadlocks manifest themselves as no output
being produced because all processes are sleeping (hit ctrl-p a few
times). Your kernel might also suffer kernel panics, because your
changes violate invariants. You may have to iterate a few times to
get a good design and implementation.
<h2>umount</h2>
<p>Once your kernel passes usertests and test0 of mounttest, implement
umount. The main challenge is that umount of a file system should
fail if the file system is still in use; that is, if there is an
inode on the mounted device that has a <tt>ref > 0</tt>.
Furthermore, this test and unmounting should be an atomic
operation. (Hint: lock the inode cache.) Make sure your kernel
passes test1 of mounttest.
<p>Test2 of mounttest stresses <tt>namex</tt> more; if you have done
everything right above, your kernel should pass it. Test3 tests
concurrent mount/unmounts with file creation.
<h2>crash safety</h2>
<p>One of the main goals of the file system is to provide crash
safety: if there is a power failure during a file system operation,
xv6 should recover correctly. It is difficult to introduce power
failure at the critical steps of logging; instead, we added a system
call that causes a kernel panic after committing an operation but
before installing the operation. Test4 with crashtest tests if your
xv6 recovers the mounted disk correctly.
</body>
</html>
<h2>Optional challenges</h2>
<p>Modify xv6 so that init mounts the first disk on the root inode.
This will allow you to remove some code specific for the first disk
from the kernel.
<p>Support mounts on top of mounts.

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<html>
<head>
<title>Lab: xv6 lazy page allocation</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: xv6 lazy page allocation</h1>
<p>
One of the many neat tricks an O/S can play with page table hardware
is lazy allocation of heap memory. Xv6 applications ask the kernel for
heap memory using the sbrk() system call. In the kernel we've given
you, sbrk() allocates physical memory and maps it into the process's
virtual address space. There are programs that allocate memory but
never use it, for example to implement large sparse arrays.
Sophisticated kernels delay allocation of each page of memory until
the application tries to use that page -- as signaled by a page fault.
You'll add this lazy allocation feature to xv6 in this lab.
<h2>Part One: Eliminate allocation from sbrk()</h2>
Your first task is to delete page allocation from the sbrk(n) system
call implementation, which is the function sys_sbrk() in sysproc.c. The
sbrk(n) system call grows the process's memory size by n bytes, and
then returns the start of the newly allocated region (i.e., the old
size). Your new sbrk(n) should just increment the process's size
(myproc()->sz) by n and return the old size. It should not allocate memory
-- so you should delete the call to growproc() (but you still need to
increase the process's size!).
<p>
Try to guess what the result of this modification will be: what will
break?
<p>
Make this modification, boot xv6, and type <tt>echo hi</tt> to the shell.
You should see something like this:
<pre>
init: starting sh
$ echo hi
usertrap(): unexpected scause 0x000000000000000f pid=3
sepc=0x00000000000011dc stval=0x0000000000004008
va=0x0000000000004000 pte=0x0000000000000000
panic: unmappages: not mapped
</pre>
The "usertrap(): ..." message is from the user trap handler in trap.c;
it has caught an exception that it does not know how to handle. Make
sure you understand why this page fault occurs. The "stval=0x0..04008"
indicates that the virtual address that caused the page fault is
0x4008.
<h2>Part Two: Lazy allocation</h2>
Modify the code in trap.c to respond to a page fault from user space
by mapping a newly-allocated page of physical memory at the faulting
address, and then returning back to user space to let the process
continue executing. You should add your code just before
the <tt>printf</tt> call that produced the "usertrap(): ..."
message.
<p>
Hint: look at the printf arguments to see how to find the virtual
address that caused the page fault.
<p>
Hint: steal code from allocuvm() in vm.c, which is what sbrk()
calls (via growproc()).
<p>
Hint: use PGROUNDDOWN(va) to round the faulting virtual address
down to a page boundary.
<p>
Hint: <tt>usertrapret()</tt> in order to avoid
the <tt>printf</tt> and the <tt>myproc()->killed&nbsp;=&nbsp;1</tt>.
<p>
Hint: you'll need to call mappages().
<p>Hint: you can check whether a fault is a page fault by r_scause()
is 13 or 15 in trap().
<p>Hint: modify unmappages() to not free pages that aren't mapped.
<p>Hint: if the kernel crashes, look up sepc in kernel/kernel.asm
<p>Hint: if you see the error "imcomplete type proc", include "proc.h"
(and "spinlock.h").
<p>Hint: the first test in sbrk() allocates something large, this
should succeed now.
<p>
If all goes well, your lazy allocation code should result in <tt>echo
hi</tt> working. You should get at least one page fault (and thus lazy
allocation) in the shell, and perhaps two.
<p>If you have the basics working, now turn your implementation into
one that handles the corner cases too:
<ul>
<li> Handle negative sbrk() arguments. sbrktest() in usertests will
tests this.
<li> Handle fork correctly. sbrktst() will test this.
<li> Make sure that kernel use of not-yet-allocated user addresses
works; for example, if a program passes an sbrk()-allocated
address to write(). sbrktest() will test this.
<li> Handle out of memory correctly. sbrktst() will test this.
<li> Handle faults on the invalid page below the stack. stacktest()
in usertests will tests this.
</ul>
<p>Run all tests in usertests() to make sure your solution doesn't
break other tests.
<p>
<div class="question">
<p><b>Submit</b>: The code that you added to trap.c in a file named <em>hwN.c</em> where <em>N</em> is the homework number as listed on the schedule.
</div>
</body>
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<html>
<head>
<title>Lab: locks</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: locks</h1>
<p>In this lab you will try to avoid lock contention for certain
workloads.
<h2>lock contention</h2>
<p>The program user/kalloctest stresses xv6's memory allocator: three
processes grow and shrink there address space, which will results in
many calls to <tt>kalloc</tt> and <tt>kfree</tt>,
respectively. <tt>kalloc</tt> and <tt>kfree</tt>
obtain <tt>kmem.lock</tt>. To see if there is lock contention for
<tt>kmem.lock</tt> replace the call to <tt>acquire</tt>
in <tt>kalloc</tt> with the following code:
<pre>
while(!tryacquire(&kmem.lock)) {
printf("!");
}
</pre>
<p><tt>tryacquire</tt> tries to acquire <tt>kmem.lock</tt>: if the
lock is taking it returns false (0); otherwise, it returns true (1)
and with the lock acquired. Your first job is to
implement <tt>tryacquire</tt> in kernel/spinlock.c.
<p>A few hints:
<ul>
<li>look at <tt>acquire</tt>.
<li>don't forget to restore interrupts when acquision fails
<li>Add tryacquire's signature to defs.h.
</ul>
<p>Run usertests to see if you didn't break anything. Note that
usertests never prints "!"; there is never contention
for <tt>kmem.lock</tt>. The caller is always able to immediately
acquire the lock and never has to wait because some other process
has the lock.
<p>Now run kalloctest. You should see quite a number of "!" on the
console. kalloctest causes many processes to contend on
the <tt>kmem.lock</tt>. This lock contention is a bit artificial,
because qemu is simulating 3 processors, but it is likely on real
hardware, there would be contention too.
<h2>Removing lock contention</h2>
<p>The root cause of lock contention in kalloctest is that there is a
single free list, protected by a single lock. To remove lock
contention, you will have to redesign the memory allocator to avoid
a single lock and list. The basic idea is to maintain a free list
per CPU, each list with its own lock. Allocations and frees on each
CPU can run in parallel, because each CPU will operate on a
different list.
<p> The main challenge will be to deal with the case that one CPU runs
out of memory, but another CPU has still free memory; in that case,
the one CPU must "steal" part of the other CPU's free list.
Stealing may introduce lock contention, but that may be acceptable
because it may happen infrequently.
<p>Your job is to implement per-CPU freelists and stealing when one
CPU is out of memory. Run kalloctest() to see if your
implementation has removed lock contention.
<p>Some hints:
<ul>
<li>You can use the constant <tt>NCPU</tt> in kernel/param.h
<li>Let <tt>freerange</tt> give all free memory to the CPU
running <tt>freerange</tt>.
<li>The function <tt>cpuid</tt> returns the current core, but note
that you can use it when interrupts are turned off and so you will
need to turn on/off interrupts in your solution.
</ul>
<p>Run usertests to see if you don't break anything.
<h2>More scalabale bcache lookup</h2>
<p>Several processes reading different files repeatedly will
bottleneck in the buffer cache, bcache, in bio.c. Replace the
acquire in <tt>bget</tt> with
<pre>
while(!tryacquire(&bcache.lock)) {
printf("!");
}
</pre>
and run test0 from bcachetest and you will see "!"s.
<p>Modify <tt>bget</tt> so that a lookup for a buffer that is in the
bcache doesn't need to acquire <tt>bcache.lock</tt>. This is more
tricky than the kalloc assignment, because bcache buffers are truly
shared among processes. You must maintain the invariant that a
buffer is only once in memory.
<p> There are several races that <tt>bcache.lock</tt> protects
against, including:
<ul>
<li>A <tt>brelse</tt> may set <tt>b->ref</tt> to 0,
while concurrent <tt>bget</tt> is incrementing it.
<li>Two <tt>bget</tt> may see <tt>b->ref = 0</tt> and one may re-use
the buffer, while the other may replaces it with another block.
<li>A concurrent <tt>brelse</tt> modifies the list
that <tt>bget</tt> traverses.
</ul>
<p>A challenge is testing whether you code is still correct. One way
to do is to artificially delay certain operations
using <tt>sleepticks</tt>. <tt>test1</tt> trashes the buffer cache
and exercises more code paths.
<p>Here are some hints:
<ul>
<li>Read the description of buffer cache in the xv6 book (Section 7.2).
<li>Use a simple design: i.e., don't design a lock-free implementation.
<li>Use a simple hash table with locks per bucket.
<li>Searching in hash table for a buffer and allocating an entry
for that buffer when the buffer is not found must be atomic.
<li>It is fine to acquire <tt>bcache.lock</tt> in <tt>brelse</tt>
to update the LRU/MRU list.
</ul>
<p>Check that your implementation has less contention
on <tt>test0</tt>
<p>Make sure your implementation passes bcachetest and usertests.
<p>Optional:
<ul>
<li>make the buffer cache more scalable (e.g., avoid taking
out <tt>bcache.lock</tt> on <tt>brelse</tt>).
<li>make lookup lock-free (Hint: use gcc's <tt>__sync_*</tt>
functions.) How do you convince yourself that your implementation is correct?
</ul>
</body>
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<html>
<head>
<title>Lab: mmap</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: mmap</h1>
<p>In this lab you will use </tt>mmap</tt> on Linux to demand-page a
very large table and add memory-mapped files to xv6.
<h2>Using mmap on Linux</h2>
<p>This assignment will make you more familiar with how to manage virtual memory
in user programs using the Unix system call interface. You can do this
assignment on any operating system that supports the Unix API (a Linux Athena
machine, your laptop with Linux or MacOS, etc.).
<p>Download the <a href="mmap.c">mmap homework assignment</a> and look
it over. The program maintains a very large table of square root
values in virtual memory. However, the table is too large to fit in
physical RAM. Instead, the square root values should be computed on
demand in response to page faults that occur in the table's address
range. Your job is to implement the demand faulting mechanism using a
signal handler and UNIX memory mapping system calls. To stay within
the physical RAM limit, we suggest using the simple strategy of
unmapping the last page whenever a new page is faulted in.
<p>To compile <tt>mmap.c</tt>, you need a C compiler, such as gcc. On Athena,
you can type:
<pre>
$ add gnu
</pre>
Once you have gcc, you can compile mmap.c as follows:
<pre>
$ gcc mmap.c -lm -o mmap
</pre>
Which produces a <tt>mmap</tt> file, which you can run:
<pre>
$ ./mmap
page_size is 4096
Validating square root table contents...
oops got SIGSEGV at 0x7f6bf7fd7f18
</pre>
<p>When the process accesses the square root table, the mapping does not exist
and the kernel passes control to the signal handler code in
<tt>handle_sigsegv()</tt>. Modify the code in <tt>handle_sigsegv()</tt> to map
in a page at the faulting address, unmap a previous page to stay within the
physical memory limit, and initialize the new page with the correct square root
values. Use the function <tt>calculate_sqrts()</tt> to compute the values.
The program includes test logic that verifies if the contents of the
square root table are correct. When you have completed your task
successfully, the process will print &ldquo;All tests passed!&rdquo;.
<p>You may find that the man pages for mmap() and munmap() are helpful references.
<pre>
$ man mmap
$ man munmap
</pre>
<h2>Implement memory-mapped files in xv6</h2>
<p>In this assignment you will implement memory-mapped files in xv6.
The test program <tt>mmaptest</tt> tells you what should work.
<p>Here are some hints about how you might go about this assignment:
<ul>
<li>Start with adding the two systems calls to the kernel, as you
done for other systems calls (e.g., <tt>sigalarm</tt>), but
don't implement them yet; just return an
error. run <tt>mmaptest</tt> to observe the error.
<li>Keep track for each process what <tt>mmap</tt> has mapped.
You will need to allocate a <tt>struct vma</tt> to record the
address, length, permissions, etc. for each virtual memory area
(VMA) that maps a file. Since the xv6 kernel doesn't have a
memory allocator in the kernel, you can use the same approach has
for <tt>struct file</tt>: have a global array of <tt>struct
vma</tt>s and have for each process a fixed-sized array of VMAs
(like the file descriptor array).
<li>Implement <tt>mmap</tt>: allocate a VMA, add it to the process's
table of VMAs, fill in the VMA, and find a hole in the process's
address space where you will map the file. You can assume that no
file will be bigger than 1GB. The VMA will contain a pointer to
a <tt>struct file</tt> for the file being mapped; you will need to
increase the file's reference count so that the structure doesn't
disappear when the file is closed (hint:
see <tt>filedup</tt>). You don't have worry about overlapping
VMAs. Run <tt>mmaptest</tt>: the first <tt>mmap</tt> should
succeed, but the first access to the mmaped- memory will fail,
because you haven't updated the page fault handler.
<li>Modify the page-fault handler from the lazy-allocation and COW
labs to call a VMA function that handles page faults in VMAs.
This function allocates a page, reads a 4KB from the mmap-ed
file into the page, and maps the page into the address space of
the process. To read the page, you can use <tt>readi</tt>,
which allows you to specify an offset from where to read in the
file (but you will have to lock/unlock the inode passed
to <tt>readi</tt>). Don't forget to set the permissions correctly
on the page. Run <tt>mmaptest</tt>; you should get to the
first <tt>munmap</tt>.
<li>Implement <tt>munmap</tt>: find the <tt>struct vma</tt> for
the address and unmap the specified pages (hint:
use <tt>uvmunmap</tt>). If <tt>munmap</tt> removes all pages
from a VMA, you will have to free the VMA (don't forget to
decrement the reference count of the VMA's <tt>struct
file</tt>); otherwise, you may have to shrink the VMA. You can
assume that <tt>munmap</tt> will not split a VMA into two VMAs;
that is, we don't unmap a few pages in the middle of a VMA. If
an unmapped page has been modified and the file is
mapped <tt>MAP_SHARED</tt>, you will have to write the page back
to the file. RISC-V has a dirty bit (<tt>D</tt>) in a PTE to
record whether a page has ever been written too; add the
declaration to kernel/riscv.h and use it. Modify <tt>exit</tt>
to call <tt>munmap</tt> for the process's open VMAs.
Run <tt>mmaptest</tt>; you should <tt>mmaptest</tt>, but
probably not <tt>forktest</tt>.
<li>Modify <tt>fork</tt> to copy VMAs from parent to child. Don't
forget to increment reference count for a VMA's <tt>struct
file</tt>. In the page fault handler of the child, it is OK to
allocate a new page instead of sharing the page with the
parent. The latter would be cooler, but it would require more
implementation work. Run <tt>mmaptest</tt>; make sure you pass
both <tt>mmaptest</tt> and <tt>forktest</tt>.
</ul>
<p>Run usertests to make sure you didn't break anything.
<p>Optional challenges:
<ul>
<li>If two processes have the same file mmap-ed (as
in <tt>forktest</tt>), share their physical pages. You will need
reference counts on physical pages.
<li>The solution above allocates a new physical page for each page
read from the mmap-ed file, even though the data is also in kernel
memory in the buffer cache. Modify your implementation to mmap
that memory, instead of allocating a new page. This requires that
file blocks be the same size as pages (set <tt>BSIZE</tt> to
4096). You will need to pin mmap-ed blocks into the buffer cache.
You will need worry about reference counts.
<li>Remove redundancy between your implementation for lazy
allocation and your implementation of mmapp-ed files. (Hint:
create an VMA for the lazy allocation area.)
<li>Modify <tt>exec</tt> to use a VMA for different sections of
the binary so that you get on-demand-paged executables. This will
make starting programs faster, because <tt>exec</tt> will not have
to read any data from the file system.
<li>Implement on-demand paging: don't keep a process in memory,
but let the kernel move some parts of processes to disk when
physical memory is low. Then, page in the paged-out memory when
the process references it. Port your linux program from the first
assignment to xv6 and run it.
</ul>
</body>
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<html>
<head>
<title>Lab: Alarm and uthread</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: Alarm and uthread</h1>
This lab will familiarize you with the implementation of system calls
and switching between threads of execution. In particular, you will
implement new system calls (<tt>sigalarm</tt> and <tt>sigreturn</tt>)
and switching between threads in a user-level thread package.
<h2>Warmup: RISC-V assembly</h2>
<p>For this lab it will be important to understand a bit of RISC-V assembly.
<p>Add a file user/call.c with the following content, modify the
Makefile to add the program to the user programs, and compile (make
fs.img). The Makefile also produces a binary and a readable
assembly a version of the program in the file user/call.asm.
<pre>
#include "kernel/param.h"
#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"
int g(int x) {
return x+3;
}
int f(int x) {
return g(x);
}
void main(void) {
printf(1, "%d %d\n", f(8)+1, 13);
exit();
}
</pre>
<p>Read through user/call.asm and understand it. The instruction manual
for RISC-V is in the doc directory (doc/riscv-spec-v2.2.pdf). Here
are some questions that you should answer for yourself:
<ul>
<li>Which registers contain arguments to functions? Which
register holds 13 in the call to <tt>printf</tt>? Which register
holds the second argument? Which register holds the third one? Etc.
<li>Where is the function call to <tt>f</tt> from main? Where
is the call to <tt>g</tt>?
(Hint: the compiler may inline functions.)
<li>At what address is the function <tt>printf</tt> located?
<li>What value is in the register <tt>ra</tt> just after the <tt>jalr</tt>
to <tt>printf</tt> in <tt>main</tt>?
</ul>
<h2>Warmup: system call tracing</h2>
<p>In this exercise you will modify the xv6 kernel to print out a line
for each system call invocation. It is enough to print the name of the
system call and the return value; you don't need to print the system
call arguments.
<p>
When you're done, you should see output like this when booting
xv6:
<pre>
...
fork -> 2
exec -> 0
open -> 3
close -> 0
$write -> 1
write -> 1
</pre>
<p>
That's init forking and execing sh, sh making sure only two file descriptors are
open, and sh writing the $ prompt. (Note: the output of the shell and the
system call trace are intermixed, because the shell uses the write syscall to
print its output.)
<p> Hint: modify the syscall() function in kernel/syscall.c.
<p>Run the xv6 programs you wrote in earlier labs and inspect the system call
trace. Are there many system calls? Which system calls correspond
to code in the applications you wrote?
<p>Optional: print the system call arguments.
<h2>Alarm</h2>
<p>
In this exercise you'll add a feature to xv6 that periodically alerts
a process as it uses CPU time. This might be useful for compute-bound
processes that want to limit how much CPU time they chew up, or for
processes that want to compute but also want to take some periodic
action. More generally, you'll be implementing a primitive form of
user-level interrupt/fault handlers; you could use something similar
to handle page faults in the application, for example.
<p>
You should add a new <tt>sigalarm(interval, handler)</tt> system call.
If an application calls <tt>sigalarm(n, fn)</tt>, then after every
<tt>n</tt> "ticks" of CPU time that the program consumes, the kernel
should cause application function
<tt>fn</tt> to be called. When <tt>fn</tt> returns, the application
should resume where it left off. A tick is a fairly arbitrary unit of
time in xv6, determined by how often a hardware timer generates
interrupts.
<p>
You'll find a file <tt>user/alarmtest.c</tt> in your xv6
repository. Add it to the Makefile. It won't compile correctly
until you've added <tt>sigalarm</tt> and <tt>sigreturn</tt>
system calls (see below).
<p>
<tt>alarmtest</tt> calls <tt>sigalarm(2, periodic)</tt> in <tt>test0</tt> to
ask the kernel to force a call to <tt>periodic()</tt> every 2 ticks,
and then spins for a while.
You can see the assembly
code for alarmtest in user/alarmtest.asm, which may be handy
for debugging.
When you've finished the lab,
<tt>alarmtest</tt> should produce output like this:
<pre>
$ alarmtest
test0 start
......................................alarm!
test0 passed
test1 start
..alarm!
..alarm!
..alarm!
.alarm!
..alarm!
..alarm!
..alarm!
..alarm!
..alarm!
..alarm!
test1 passed
$
</pre>
<p>The main challenge will be to arrange that the handler is invoked
when the process's alarm interval expires. You'll need to modify
usertrap() in kernel/trap.c so that when a
process's alarm interval expires, the process executes
the handler. How can you do that? You will need to understand
how system calls work (i.e., the code in kernel/trampoline.S
and kernel/trap.c). Which register contains the address to which
system calls return?
<p>Your solution will be only a few lines of code, but it may be tricky to
get it right.
We'll test your code with the version of alarmtest.c in the original
repository; if you modify alarmtest.c, make sure your kernel changes
cause the original alarmtest to pass the tests.
<h3>test0: invoke handler</h3>
<p>Get started by modifying the kernel to jump to the alarm handler in
user space, which will cause test0 to print "alarm!". Don't worry yet
what happens after the "alarm!" output; it's OK for now if your
program crashes after printing "alarm!". Here are some hints:
<ul>
<li>You'll need to modify the Makefile to cause <tt>alarmtest.c</tt>
to be compiled as an xv6 user program.
<li>The right declarations to put in <tt>user/user.h</tt> are:
<pre>
int sigalarm(int ticks, void (*handler)());
int sigreturn(void);
</pre>
<li>Update user/sys.pl (which generates user/usys.S),
kernel/syscall.h, and kernel/syscall.c
to allow <tt>alarmtest</tt> to invoke the sigalarm and
sigreturn system calls.
<li>For now, your <tt>sys_sigreturn</tt> should just return zero.
<li>Your <tt>sys_sigalarm()</tt> should store the alarm interval and
the pointer to the handler function in new fields in the <tt>proc</tt>
structure, defined in <tt>kernel/proc.h</tt>.
<li>You'll need to keep track of how many ticks have passed since the
last call (or are left until the next call) to a process's alarm
handler; you'll need a new field in <tt>struct&nbsp;proc</tt> for this
too. You can initialize <tt>proc</tt> fields in <tt>allocproc()</tt>
in <tt>proc.c</tt>.
<li>Every tick, the hardware clock forces an interrupt, which is handled
in <tt>usertrap()</tt>; you should add some code here.
<li>You only want to manipulate a process's alarm ticks if there's a a
timer interrupt; you want something like
<pre>
if(which_dev == 2) ...
</pre>
<li>Only invoke the alarm function if the process has a
timer outstanding. Note that the address of the user's alarm
function might be 0 (e.g., in alarmtest.asm, <tt>periodic</tt> is at
address 0).
<li>It will be easier to look at traps with gdb if you tell qemu to
use only one CPU, which you can do by running
<pre>
make CPUS=1 qemu
</pre>
<li>You've succeeded if alarmtest prints "alarm!".
</ul>
<h3>test1(): resume interrupted code</h3>
Chances are that alarmtest crashes at some point after it prints
"alarm!". Depending on how your solution works, that point may be in
test0, or it may be in test1. Crashes are likely caused
by the alarm handler (<tt>periodic</tt> in alarmtest.c) returning
to the wrong point in the user program.
<p>
Your job now is to ensure that, when the alarm handler is done,
control returns to
the instruction at which the user program was originally
interrupted by the timer interrupt. You must also ensure that
the register contents are restored to values they held
at the time of the interrupt, so that the user program
can continue undisturbed after the alarm.
<p>Your solution is likely to require you to save and restore
registers---what registers do you need to save and restore to resume
the interrupted code correctly? (Hint: it will be many).
Several approaches are possible; for this lab you should make
the <tt>sigreturn</tt> system call
restore registers and return to the original
interrupted user instruction.
The user-space alarm handler
calls sigreturn when it is done.
Some hints:
<ul>
<li>Have <tt>usertrap</tt> save enough state in
<tt>struct proc</tt> when the timer goes off
that <tt>sigreturn</tt> can correctly return to the
interrupted user code.
<li>Prevent re-entrant calls to the handler----if a handler hasn't
returned yet, the kernel shouldn't call it again.
</ul>
<p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to
make sure you didn't break any other parts of the kernel.
<h2>Uthread: switching between threads</h2>
<p>Download <a href="uthread.c">uthread.c</a> and <a
href="uthread_switch.S">uthread_switch.S</a> into your xv6 directory.
Make sure <tt>uthread_switch.S</tt> ends with <tt>.S</tt>, not
<tt>.s</tt>. Add the
following rule to the xv6 Makefile after the _forktest rule:
<pre>
$U/_uthread: $U/uthread.o $U/uthread_switch.o
$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o $U/_uthread $U/uthread.o $U/uthread_switch.o $(ULIB)
$(OBJDUMP) -S $U/_uthread > $U/uthread.asm
</pre>
Make sure that the blank space at the start of each line is a tab,
not spaces.
<p>
Add <tt>_uthread</tt> in the Makefile to the list of user programs defined by UPROGS.
<p>Run xv6, then run <tt>uthread</tt> from the xv6 shell. The xv6 kernel will print an error message about <tt>uthread</tt> encountering a page fault.
<p>Your job is to complete <tt>uthread_switch.S</tt>, so that you see output similar to
this (make sure to run with CPUS=1):
<pre>
~/classes/6828/xv6$ make CPUS=1 qemu
...
$ uthread
my thread running
my thread 0x0000000000002A30
my thread running
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
...
my thread 0x0000000000002A88
my thread 0x0000000000004A98
my thread: exit
my thread: exit
thread_schedule: no runnable threads
$
</pre>
<p><tt>uthread</tt> creates two threads and switches back and forth between
them. Each thread prints "my thread ..." and then yields to give the other
thread a chance to run.
<p>To observe the above output, you need to complete <tt>uthread_switch.S</tt>, but before
jumping into <tt>uthread_switch.S</tt>, first understand how <tt>uthread.c</tt>
uses <tt>uthread_switch</tt>. <tt>uthread.c</tt> has two global variables
<tt>current_thread</tt> and <tt>next_thread</tt>. Each is a pointer to a
<tt>thread</tt> structure. The thread structure has a stack for a thread and a
saved stack pointer (<tt>sp</tt>, which points into the thread's stack). The
job of <tt>uthread_switch</tt> is to save the current thread state into the
structure pointed to by <tt>current_thread</tt>, restore <tt>next_thread</tt>'s
state, and make <tt>current_thread</tt> point to where <tt>next_thread</tt> was
pointing to, so that when <tt>uthread_switch</tt> returns <tt>next_thread</tt>
is running and is the <tt>current_thread</tt>.
<p>You should study <tt>thread_create</tt>, which sets up the initial stack for
a new thread. It provides hints about what <tt>uthread_switch</tt> should do.
Note that <tt>thread_create</tt> simulates saving all callee-save registers
on a new thread's stack.
<p>To write the assembly in <tt>thread_switch</tt>, you need to know how the C
compiler lays out <tt>struct thread</tt> in memory, which is as
follows:
<pre>
--------------------
| 4 bytes for state|
--------------------
| stack size bytes |
| for stack |
--------------------
| 8 bytes for sp |
-------------------- <--- current_thread
......
......
--------------------
| 4 bytes for state|
--------------------
| stack size bytes |
| for stack |
--------------------
| 8 bytes for sp |
-------------------- <--- next_thread
</pre>
The variables <tt>&next_thread</tt> and <tt>&current_thread</tt> each
contain the address of a pointer to <tt>struct thread</tt>, and are
passed to <tt>thread_switch</tt>. The following fragment of assembly
will be useful:
<pre>
ld t0, 0(a0)
sd sp, 0(t0)
</pre>
This saves <tt>sp</tt> in <tt>current_thread->sp</tt>. This works because
<tt>sp</tt> is at
offset 0 in the struct.
You can study the assembly the compiler generates for
<tt>uthread.c</tt> by looking at <tt>uthread.asm</tt>.
<p>To test your code it might be helpful to single step through your
<tt>uthread_switch</tt> using <tt>riscv64-linux-gnu-gdb</tt>. You can get started in this way:
<pre>
(gdb) file user/_uthread
Reading symbols from user/_uthread...
(gdb) b *0x230
</pre>
0x230 is the address of uthread_switch (see uthread.asm). When you
compile it may be at a different address, so check uthread_asm.
You may also be able to type "b uthread_switch". <b>XXX This doesn't work
for me; why?</b>
<p>The breakpoint may (or may not) be triggered before you even run
<tt>uthread</tt>. How could that happen?
<p>Once your xv6 shell runs, type "uthread", and gdb will break at
<tt>thread_switch</tt>. Now you can type commands like the following to inspect
the state of <tt>uthread</tt>:
<pre>
(gdb) p/x *next_thread
$1 = {sp = 0x4a28, stack = {0x0 (repeats 8088 times),
0x68, 0x1, 0x0 <repeats 102 times>}, state = 0x1}
</pre>
What address is <tt>0x168</tt>, which sits on the bottom of the stack
of <tt>next_thread</tt>?
With "x", you can examine the content of a memory location
<pre>
(gdb) x/x next_thread->sp
0x4a28 <all_thread+16304>: 0x00000168
</pre>
Why does that print <tt>0x168</tt>?
<h3>Optional challenges</h3>
<p>The user-level thread package interacts badly with the operating system in
several ways. For example, if one user-level thread blocks in a system call,
another user-level thread won't run, because the user-level threads scheduler
doesn't know that one of its threads has been descheduled by the xv6 scheduler. As
another example, two user-level threads will not run concurrently on different
cores, because the xv6 scheduler isn't aware that there are multiple
threads that could run in parallel. Note that if two user-level threads were to
run truly in parallel, this implementation won't work because of several races
(e.g., two threads on different processors could call <tt>thread_schedule</tt>
concurrently, select the same runnable thread, and both run it on different
processors.)
<p>There are several ways of addressing these problems. One is
using <a href="http://en.wikipedia.org/wiki/Scheduler_activations">scheduler
activations</a> and another is to use one kernel thread per
user-level thread (as Linux kernels do). Implement one of these ways
in xv6. This is not easy to get right; for example, you will need to
implement TLB shootdown when updating a page table for a
multithreaded user process.
<p>Add locks, condition variables, barriers,
etc. to your thread package.
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<html>
<head>
<title>Lab: xv6</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: xv6</h1>
This lab makes you familiar with xv6 and its system calls.
<h2>Boot xv6</h2>
<p>Login to Athena (e.g., ssh -X athena.dialup.mit.edu) and attach the course
locker: (You must run this command every time you log in; or add it to your
~/.environment file.)
<pre>
$ add -f 6.828
</pre>
<p>Fetch the xv6 source:
<pre>
$ mkdir 6.828
$ cd 6.828
$ git clone git://github.com/mit-pdos/xv6-riscv.git
Cloning into 'xv6-riscv'...
...
$
</pre>
<p>XXX pointer to an update tools page
<p>Build xv6 on Athena:
<pre>
$ cd xv6-public
$ makeriscv64-linux-gnu-gcc -c -o kernel/entry.o kernel/entry.S
riscv64-linux-gnu-gcc -Wall -Werror -O -fno-omit-frame-pointer -ggdb -MD -mcmodel=medany -ffreestanding -fno-common -nostdlib -mno-relax -I. -fno-stack-protector -fno-pie -no-pie -c -o kernel/start.o kernel/start.c
...
$ make qemu
...
mkfs/mkfs fs.img README user/_cat user/_echo user/_forktest user/_grep user/_init user/_kill user/_ln user/_ls user/_mkdir user/_rm user/_sh user/_stressfs user/_usertests user/_wc user/_zombie user/_cow
nmeta 46 (boot, super, log blocks 30 inode blocks 13, bitmap blocks 1) blocks 954 total 1000
balloc: first 497 blocks have been allocated
balloc: write bitmap block at sector 45
qemu-system-riscv64 -machine virt -kernel kernel/kernel -m 3G -smp 3 -nographic -drive file=fs.img,if=none,format=raw,id=x0 -device virtio-blk-device,drive=x0,bus=virtio-mmio-bus.0
hart 0 starting
hart 2 starting
hart 1 starting
init: starting sh
$
</pre>
<p>
If you type <tt>ls</tt> at the prompt, you should output similar to the following:
<pre>
$ ls
. 1 1 1024
.. 1 1 1024
README 2 2 2181
cat 2 3 21024
echo 2 4 19776
forktest 2 5 11456
grep 2 6 24512
init 2 7 20656
kill 2 8 19856
ln 2 9 19832
ls 2 10 23280
mkdir 2 11 19952
rm 2 12 19936
sh 2 13 38632
stressfs 2 14 20912
usertests 2 15 106264
wc 2 16 22160
zombie 2 17 19376
cow 2 18 27152
console 3 19 0
</pre>
These are the programs/files that <tt>mkfs</tt> includes in the
initial file system. You just ran one of them: <tt>ls</tt>.
<h2>sleep</h2>
<p>Implement the UNIX program sleep for xv6; your sleep should pause
for a user-specified number of ticks.
<p>Some hints:
<ul>
<li>Look at some of the other programs in <tt>user/</tt> to see
how you can obtain the command-line arguments passed to a program. If the user
forgets to pass an argument, sleep should print an error message.
<li>The command-line argument is passed as a string; you can convert it to an
integer using <tt>atoi</tt> (see user/ulib.c).
<li>Use the system call <tt>sleep</tt> (see user/usys.S and kernel/sysproc.c).
<li>Make sure <tt>main</tt> calls <tt>exit()</tt> in order to exit
your program.
<li>Add the program to <tt>UPROGS</tt> in Makefile and compile
user programs by typing <tt>make fs.img</tt>.
</ul>
<p>Run the program from the xv6 shell:
<pre>
$ make qemu
...
init: starting sh
$ sleep 10
(waits for a little while)
$
</pre>
<p>Optional: write an uptime program that prints the uptime in terms
of ticks using the <tt>uptime</tt> system call.
<h2>pingpong</h2>
<p> Write a program that uses UNIX system calls to ``ping-pong'' a
byte between two processes over a pair of pipes, one for each
direction. The parent sends by writing a byte to <tt>fd[1]</tt> and
the child receives it by reading from <tt>fd[0]</tt>. After
receiving a byte from parent, the child responds with its own byte
by writing to <tt>fd[1]</tt>, which the parent then reads.
<p>Some hints:
<ul>
<li>Use <tt>pipe</tt> to create a pipe.
<li>Use <tt>fork</tt> to create a child.
<li>Use <tt>read</tt> to read from the pipe, and <tt>write</tt> to write to the pipe.
</ul>
<h2>primes</h2>
<p>Write a concurrent version of prime sieve using pipes. This idea
is due to Doug McIlroy, inventor of Unix pipes. The picture
halfway down <a href="http://swtch.com/~rsc/thread/">the page</a>
and the text surrounding it explain how to do it.
<p>Your goal is to use <tt>pipe</tt> and <tt>fork</tt> to set up
the pipeline. The first process feeds the numbers 2 through 35
into the pipeline. For each prime number, you will arrange to
create one process that reads from its left neighbor over a pipe
and writes to its right neighbor over another pipe. Since xv6 has
limited number of file descriptors and processes, the first
process can stop at 35.
<p>Some hints:
<ul>
<li>Be careful to close file descriptors that a process doesn't
need, because otherwise your program will run xv6 out of resources
before the first process reaches 35.
<li>Once the first process reach 35, you should arrange that the
pipeline terminates cleanly (Hint: read will return an end-of-file
when the write-side of the pipe is closed).
</ul>
<h2>find</h2>
<p>Write a simple version of the UNIX find program: find all the files
in a directory tree whose name matches a string. For example if the
file system contains a file <tt>a/b</tt>, then running find as
follows should produce:
<pre>
$ find . b
./a/b
$
</pre>
<p>Some hints:
<ul>
<li>Look at user/ls.c to see how to read directories.
<li>Use recursion to run find in sub-directories.
<li>Don't recurse into "." and "..".
</ul>
<p>Optional: support regular expressions in name matching. Grep has some
primitive support for regular expressions.
<h2>xargs</h2>
<p>Write a simple version of the UNIX xargs program: read lines from
standard in and run a command for each line, supplying the line as
arguments to the command. The following example illustrates xarg's
behavior:
<pre>
$ xargs echo bye
hello too
bye hello too
<ctrl-d>
$
</pre>
Note that the command here is "echo bye" and the additional
arguments are "hello too", making the command "echo bye hello too",
which outputs "bye hello too".
<p>xargs and find combine well:
<pre>
find . b | xargs grep hello
</pre>
will run "grep hello" on each file named b in the directories below ".".
<p>Some hints:
<ul>
<li>Use <tt>fork</tt> and <tt>exec</tt> system call to invoke the
command on each line of input. Use <tt>wait</tt> in the parent
to wait for the child to complete running the command.
<li>Read from stdin a character at the time until the newline
character ('\n').
<li>kernel/param.h declares MAXARG, which may be useful if you need
to declare an argv.
</ul>
<h2>Optional: modify the shell</h2>
There are endless ways in which the shell could be extended. Here are
some suggestions:
<ul>
<li>Modify the shell to support wait.
<li>Modify the shell to support lists of commands, separated by ";"
<li>Modify the shell to support sub-shells by implementing "(" and ")"
<li>Modify the shell to allow users to edit the command line
</ul>
</body>
</html>