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Subject[PATCH 0/7] Reduce GFP_ATOMIC allocation failures, candidate fix V3
Date
Sorry for the long delay in posting another version. Testing is extremely
time-consuming and I wasn't getting to work on this as much as I'd have liked.

Changelog since V2
o Dropped the kswapd-quickly-notice-high-order patch. In more detailed
testing, it made latencies even worse as kswapd slept more on high-order
congestion causing order-0 direct reclaims.
o Added changes to how congestion_wait() works
o Added a number of new patches altering the behaviour of reclaim

Since 2.6.31-rc1, there have been an increasing number of GFP_ATOMIC
failures. A significant number of these have been high-order GFP_ATOMIC
failures and while they are generally brushed away, there has been a large
increase in them recently and there are a number of possible areas the
problem could be in - core vm, page writeback and a specific driver. The
bugs affected by this that I am aware of are;

[Bug #14141] order 2 page allocation failures in iwlagn
[Bug #14141] order 2 page allocation failures (generic)
[Bug #14265] ifconfig: page allocation failure. order:5, mode:0x8020 w/ e100
[No BZ ID] Kernel crash on 2.6.31.x (kcryptd: page allocation failure..)
[No BZ ID] page allocation failure message kernel 2.6.31.4 (tty-related)

The following are a series of patches that bring the behaviour of reclaim
and the page allocator more in line with 2.6.30.

Patches 1-3 should be tested first. The testing I've done shows that the
page allocator and behaviour of congestion_wait() is more in line with
2.6.30 than the vanilla kernels.

It'd be nice to have 2 more tests, applying each patch on top noting any
behaviour change. i.e. ideally there would be results for

o patches 1+2+3
o patches 1+2+3+4
o patches 1+2+3+4+5

Of course, any tests results are welcome. The rest of the mail is the
results of my own tests.

I've tested against 2.6.31 and 2.6.32-rc6. I've somewhat replicated the
problem in Bug #14141 and believe the other bugs are variations of the same
style of problem. The basic reproduction case was;

1. X86-64 AMD Phenom and X86 P4 booted with mem=512MB. Expectation is
any machine will do as long as it's 512MB for the size of workload
involved.

2. A crypted work partition and swap partition was created. On my
own setup, I gave no passphrase so it'd be easier to activate without
interaction but there are multiple options. I should have taken better
notes but the setup goes something like this;

cryptsetup create -y crypt-partition /dev/sda5
pvcreate /dev/mapper/crypt-partition
vgcreate crypt-volume /dev/mapper/crypt-partition
lvcreate -L 5G -n crypt-logical crypt-volume
lvcreate -L 2G -n crypt-swap crypt-volume
mkfs -t ext3 /dev/crypt-volume/crypt-logical
mkswap /dev/crypt-volume/crypt-swap

3. With the partition mounted on /scratch, I
cd /scratch
mkdir music
git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git linux-2.6

4. On a normal partition, I expand a tarball containing test scripts available at
http://www.csn.ul.ie/~mel/postings/latency-20091112/latency-tests-with-results.tar.gz

There are two helper programs that run as part of the test - a fake
music player and a fake gitk.

The fake music player uses rsync with bandwidth limits to start
downloading a music folder from another machine. It's bandwidth
limited to simulate playing music over NFS. I believe it generates
similar if not exact traffic to a music player. It occured to be
afterwards that if one patched ogg123 to print a line when 1/10th
of a seconds worth of music was played, it could be used as an
indirect measure of desktop interactivity and help pin down pesky
"audio skips" bug reports.

The fake gitk is based on observing roughly what gitk does using
strace. It loads all the logs into a large buffer and then builds a
very basic hash map of parent to child commits. The data is stored
because it was insufficient just to read the logs. It had to be
kept in an in-memory buffer to generate swap. It then discards the
data and does it over again in a loop for a small number of times
so the test is finite. When it processes a large number of commits,
it outputs a line to stdout so that stalls can be observed. Ideal
behaviour is that commits are read at a constant rate and latencies
look flat.

Output from the two programs is piped through another script -
latency-output. It records how far into the test it was when the
line was outputted and what the latency was since the last line
appeared. The latency should always be very smooth. Because pipes
buffer IO, they are all run by expect_unbuffered which is available
from expect-dev on Debian at least.

All the tests are driven via run-test.sh. While the tests run,
it records the kern.log to track page allocation failures, records
nr_writeback at regular intervals and tracks Page IO and Swap IO.

5. For running an actual test, a kernel is built, booted, the
crypted partition activated, lvm restarted,
/dev/crypt-volume/crypt-logical mounted on /scratch, all
swap partitions turned off and then the swap partition on
/dev/crypt-volume/crypt-swap activated. I then run run-test.sh from
the tarball

6. Run the test script

To evaluate the patches, I considered three basic metrics.

o The length of time it takes fake-gitk to complete on average
o How often and how long fake-gitk stalled for
o How long was spent in congestion_wait

All generated data is in the tarball.

On X86, the results I got were

2.6.30-0000000-force-highorder Elapsed:10:59.095 Failures:0
2.6.31-0000000-force-highorder Elapsed:11:53.505 Failures:0
2.6.31-revert-8aa7e847 Elapsed:14:01.595 Failures:0
2.6.31-0000012-pgalloc-2.6.30 Elapsed:13:32.237 Failures:0
2.6.31-0000123-congestion-both Elapsed:12:44.170 Failures:0
2.6.31-0001234-kswapd-quick-recheck Elapsed:10:35.327 Failures:0
2.6.31-0012345-adjust-priority Elapsed:11:02.995 Failures:0
2.6.32-rc6-0000000-force-highorder Elapsed:18:18.562 Failures:0
2.6.32-rc6-revert-8aa7e847 Elapsed:10:29.278 Failures:0
2.6.32-rc6-0000012-pgalloc-2.6.30 Elapsed:13:32.393 Failures:0
2.6.32-rc6-0000123-congestion-both Elapsed:14:55.265 Failures:0
2.6.32-rc6-0001234-kswapd-quick-recheck Elapsed:13:35.628 Failures:0
2.6.32-rc6-0012345-adjust-priority Elapsed:12:41.278 Failures:0
The 0000000-force-highorder is a vanilla kernel patched so that network
receive always results in an order-2 allocation. This machine wasn't
suffering page allocation failures even under this circumstance. However,
note how slow 2.6.32-rc6 is and how much the revert helps. With the patches
applied, there is comparable performance.
Latencies were generally reduced with the patches applied. 2.6.32-rc6 was
particularly crazy with long stalls measured over the duration of the test
but has comparable latencies with 2.6.30 with the patches applied.

congestion_wait behaviour is more in line with 2.6.30 after the
patches with similar amounts of time being spent. In general,
2.6.32-rc6-0012345-adjust-priority waits for longer than 2.6.30 or the
reverted kernels did. It also waits in more instances such as inside
shrink_inactive_list() where it didn't before. Forcing behaviour like 2.6.30
resulted in good figures but I couldn't justify the patches with anything
more solid than "in tests, it behaves well even though it doesn't make a
lot of sense"

On X86-64, the results I got were

2.6.30-0000000-force-highorder Elapsed:09:48.545 Failures:0
2.6.31-0000000-force-highorder Elapsed:09:13.020 Failures:0
2.6.31-revert-8aa7e847 Elapsed:09:02.120 Failures:0
2.6.31-0000012-pgalloc-2.6.30 Elapsed:08:52.742 Failures:0
2.6.31-0000123-congestion-both Elapsed:08:59.375 Failures:0
2.6.31-0001234-kswapd-quick-recheck Elapsed:09:19.208 Failures:0
2.6.31-0012345-adjust-priority Elapsed:09:39.225 Failures:0
2.6.32-rc6-0000000-force-highorder Elapsed:19:38.585 Failures:5
2.6.32-rc6-revert-8aa7e847 Elapsed:17:21.257 Failures:0
2.6.32-rc6-0000012-pgalloc-2.6.30 Elapsed:18:56.682 Failures:1
2.6.32-rc6-0000123-congestion-both Elapsed:16:08.340 Failures:0
2.6.32-rc6-0001234-kswapd-quick-recheck Elapsed:18:11.200 Failures:7
2.6.32-rc6-0012345-adjust-priority Elapsed:21:33.158 Failures:0
Failures were down and my impression was that it was much harder to cause
failures. Performance on mainline is still not as good as 2.6.30. On
this particular machine, I was able to force performance to be in line
but not with any patch I could justify in the general case.

Latencies were slightly reduced by applying the patches against 2.6.31.
against 2.6.32-rc6, applying the patches significantly reduced the latencies
but they are still significant. I'll continue to investigate what can be
done to improve this further.

Again, congestion_wait() is more in line with 2.6.30 when the patches
are applied. Similarly to X86, almost identical behaviour can be forced
by waiting on BLK_ASYNC_BOTH for each caller to congestion_wait() in the
reclaim and allocator paths.

Bottom line, the patches made triggering allocation failures much harder
and in a number of instances and latencies are reduced when the system
is under load. I will keep looking around this area - particularly the
performance under load on 2.6.32-rc6 but with 2.6.32 almost out the door,
I am releasing what I have now.


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