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SubjectRe: [Bug #14141] order 2 page allocation failures in iwlagn
Date
Sorry for going quiet on this issue for a few days, but I have been 
spending *a lot* of time on it. I've done what amounts to 5 bisection
rounds at ~20 minutes per iteration and in total over 80 boots.

The problem with my first bisection was that there are *at least two*
changes at the root of this issue, both committed between .30 and .30-rc1.
Because of this a normal bisection will not lead to a reliable result and
even with my last effort I can only narrow it down to two different areas,
and not 100% to specific commits.

The two identified areas are:
1) a wireless merge which causes the SKB errors to appear in the first
place, but not always;
2) an mm merge which makes the SKB errors occur *much* quicker; IMHO this
is the change that also causes the regressions reported by Pekka and
Karol.

So below my results. The issue is both complex and subtle. Now it's up to
you, domain experts for both mm *and* wireless/networking, to make sense of
it all and come up with suggestions on how to proceed.

I've improved my test and it's now a lot more reliable, but there are still
timing influences. Also, because this is all merge-window stuff, I'm
hitting quite a few minor and major regressions between commits that can
affect tests.

Please study the information below carefully. I know it's long, but I think
this issue justifies that.

On Monday 05 October 2009, Frans Pop wrote:
> This looks conclusive. I tested .30 and .32-rc3 from clean reboots and
> only starting gitk. I only started music playing in the background
> (amarok) from an NFS share to ensure network activity.
>
> With .32-rc3 I got 4 SKB allocation errors while starting the *second*
> gitk instance. And the system was completely frozen with music stopped
> until gitk finished loading.

With .32-rc3, .31.1 and vanilla .31 I will get multiple SKB allocation
errors the *first time* I run the test, *every* time.

> With .30 I was able to start *three* gitk's (which meant 2 of them got
> (partially) swapped out) without any allocation errors. And with the
> system remaining relatively responsive. There was a short break in the
> music while I started the 2nd instance, but it just continued playing
> afterwards. There was also some mild latency in the mouse cursor, but
> nothing like the full desktop freeze I get with .32-rc3.

With both .30.2 and vanilla .30 I have *never* been able to get any SKB
allocation errors. No matter how often I repeat the test.

So, the start and end position are 100% reproducible. Problem is that this
changes during the bisection. At some point the test will fail (no SKB
errors) the first time I run it, but it will fail on the second or third
attempt.
Apparently at some point memory must already be fragmented (or higher
orders already used up) to some extend for the errors to trigger.

TEST METHOD
-----------
As a normal bisection (I tried 3 times...) did not lead anywhere, I had to
think of an alternative approach. I decided to start by manually selecting
merges by Linus into mainline. The advantage is that that makes the
bisection linear and makes it a lot easier to see patterns.
After narrowing down to a specific merge, I bisected (again semi-manually)
inside that merge.
Because I suspected there were multiple changes involved, I deliberately
tried to find two points:
- where do I first start seeing SKB errors at all, even if it is only at
the second or third try;
- where do I start getting SKB errors reliably on the first try.

I worked from "good" to "bad", i.e. I started at .30. The merges were not
chosen completely randomly. From the first 3 bisections I strongly
suspected the first 'net-next' merge and the first 'akpm' merge, but I did
make sure to confirm that suspicion.

TEST DESCRIPTION
----------------
The test I've ended up using is:
1) clean boot
2) start music in amarok from NFS share; use very long song to avoid file
changes and thus ensure a fluent stream of network data during the test
3) start 'gitk v2.6.29..master &' - to use up some memory
4) start first 'gitk master &' - after this all normal memory is as good as
used up, with minor swap; this never resulted in SKB errors
5) start second 'gitk master &' - this causes heavy swapping (>700 MB) and
is the real test
6) if there were no SKB errors after 5), kill the gitk processes and repeat
steps 3) to 5). I've done this up to 4 times in some cases
7) if the results are not clear or when there is doubt later, repeat from
step 1) with same kernel
Memory after initial 'gitk v2.6.29..master &':
total used free shared buffers cached
Mem: 2030776 1153008 877768 0 41572 333968
-/+ buffers/cache: 777468 1253308
Swap: 2097144 0 2097144

Memory after first 'gitk master &':
total used free shared buffers cached
Mem: 2030776 1979040 51736 0 35684 238420
-/+ buffers/cache: 1704936 325840
Swap: 2097144 21876 2075268

Memory after second 'gitk master &' (with .30.2):
total used free shared buffers cached
Mem: 2030776 2011608 19168 0 21836 92336
-/+ buffers/cache: 1897436 133340
Swap: 2097144 776160 1320984

OVERVIEW OF RESULTS
-------------------
Below I list the most relevant merges and commits. Note that they are
listed in commit order; my kernel version shows the order of testing.
For the commits I tested the test results are listed on the next line.
The first number on that line consists of the test series + the iteration
(and also identifies the kernel I used).
A "+" means I got no SKB errors, a "-" that I did get them. A "|" means I
rebooted for a second series of tests.
v2.6.30-2330-gdb8e7f1 'x86-fixes-for-linus' of linux-2.6-tip
1.1 +++ iwlagn sw-error during first test
v2.6.30-4127-g0fa2133 'merge' of powerpc (last merge before net-next-2.6)
1.2 +++
v2.6.30-5398-g2ed0e21 net-next-2.6 (mega-merge!)
1.4 +- system reboot fails after testing
v2.6.30-5517-g609106b 'merge' of powerpc
1.3 +- system reboot fails after testing
v2.6.30-5927-gf83b1e6 'for-linus' of linux1394-2.6 (last merge before akpm)
2.2 ++-
v2.6.30-6111-g517d086 'akpm'
2.1 -|-
BISECTION OF net-next-2.6 MERGE
-------------------------------
Note that this merge was based not on .30 vanilla, but partly on
v2.6.30-rc1 and partly on v2.6.30-rc6.
I think this had an influence on the latencies I saw (i.e. because some
post-rc6 bug fixes were not present it changes the general behavior of the
system during the swapping). For example: with v2.6.30-4127-g0fa2133 the
system remained more responsive (smaller music skips) than with
v2.6.30-rc1-1219-g82d0481.
I started again by testing merges, this time those by David.

v2.6.30-rc1-1219-g82d0481 'master' of wireless-next-2.6
1.5 ++++ bad latencies
v2.6.30-rc6-660-gbb803cf 'master' of net-2.6
v2.6.30-rc6-808-g45ea4ea 'master' of wireless-next-2.6
v2.6.30-rc6-850-gc649c0e 'master' of net-2.6
v2.6.30-rc6-922-g3f1f39c 'linux-2.6.31.y' of wimax
v2.6.30-rc6-999-gb2f8f75 'master' of net-2.6
v2.6.30-rc6-1028-ga8c617e 'net-next' of lksctp-dev
1.7 ++++|++++|++++
I went back to this one twice because the bisection inside the
next merge (see below) did not give a clear result.
v2.6.30-rc6-1103-gb1bc81a 'master' of wireless-next-2.6
1.8 +-
v2.6.30-rc6-1224-g84503dd 'master' of wireless-next-2.6
1.6 +-
So the problem started in the v2.6.30-rc6-1103-gb1bc81a merge.
I was unable to narrow it down to an exact commit; AFAICT the remaining
ones (between v2.6.30-rc6-1028-g8fc0fee and v2.6.30-rc6-1032-g7ba10a8) are
uninteresting. But it *must* be in this area!

For a good overview of the area, use 'gitk 3f1f39c4..b1bc81a0'.

v2.6.30-rc6-1028-g8fc0fee cfg80211: use key size constants
1.11 ++++
v2.6.30-rc6-1031-g1bb5633 iwmc3200wifi: fix printk format
1.14 +++- not quite conclusive...
v2.6.30-rc6-1032-g7ba10a8 mac80211: fix transposed min/max CW values
1.13 -
This is a bugfix for aa837ee1d from an earlier merge! Could this maybe
influence the test results in between? There are various SKB related
changes there, for example: dfbf97f3..e5b9215e.
v2.6.30-rc6-1037-g2c5b9e5 wireless: libertas: fix unaligned accesses
1.12 +-
v2.6.30-rc6-1044-g729e9c7 cfg80211: fix for duplicate userspace replies
1.10 +-
v2.6.30-rc6-1075-gc587de0 iwlwifi: unify station management
1.9 ++-|+-
v2.6.30-rc6-1076-gd14d444 iwl3945: port allow skb allocation in tasklet
I thought this was a prime candidate, but as you can see several commits
before failed too. Still worth looking at I think!
BISECTION of akpm (mm) MERGE
----------------------------
So here I went looking for "where does the test start failing on the first
try". Again, I was unable to narrow it down to a single commit.
For a good overview of the area, use 'gitk f83b1e61..517d0869'.

v2.6.30-5466-ga1dd268 mm: use alloc_pages_exact in alloc_large_system_hash
2.3 +-
v2.6.30-5478-ge9bb35d mm: setup_per_zone_inactive_ratio - fix comment and..
2.5 +-
v2.6.30-5486-g35282a2 migration: only migrate_prep() once per move_pages()
2.6 -|+|- not quite conclusive...
v2.6.30-5492-gbce7394 page-allocator: reset wmark_min and inactive ratio..
2.4 -|-
WHERE NEXT?
===========
I think the results confirm there is definitely an issue here and that my
test is reliable and consistent enough to show it. And as it currently is
the only test we have...
I hope that the info above is enough for the mm and wireless domain
experts to identify likely candidates in the areas I've identified.

The next step could be trying specific reverts or debug patches, either on
top of current git, or 2.6.31, or inside the identified areas.
I'll run anything you care to throw at me and will try to provide any
additional info you need, but at this point it's up to you.

Cheers,
FJP


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