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SubjectRe: cgroup: rmdir() does not complete
Hi.

On Thu, 26 Aug 2010 16:51:55 +0100 (BST)
Mark Hills <mark@pogo.org.uk> wrote:

> I am experiencing hung tasks when trying to rmdir() on a cgroup. One task
> spins, others queue up behind it with the following:
>
> INFO: task soaked-cgroup:27257 blocked for more than 120 seconds.
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> soaked-cgrou D ffff8800058157c0 0 27257 29411 0x00000000
> ffff88004ffffdd8 0000000000000086 ffff88004ffffda8 ffff88004ffffeb8
> 0000000000000010 ffff880119813780 ffff88004ffffd48 ffff88004fffffd8
> ffff88004fffffd8 000000000000f9b0 00000000000157c0 ffff880137693268
> Call Trace:
> [<ffffffff81115edb>] ? mntput_no_expire+0x24/0xe7
> [<ffffffff81427acd>] __mutex_lock_common+0x14d/0x1b4
> [<ffffffff81108a7c>] ? path_put+0x1d/0x22
> [<ffffffff81427b48>] __mutex_lock_slowpath+0x14/0x16
> [<ffffffff81427c4f>] mutex_lock+0x31/0x4b
> [<ffffffff8110bdf8>] do_rmdir+0x74/0x102
> [<ffffffff8110bebd>] sys_rmdir+0x11/0x13
> [<ffffffff81009b02>] system_call_fastpath+0x16/0x1b
>
> Kernel is from Fedora, 2.6.33.6. In all cases the cgroup contains no
> tasks.
>
> Commit ec64f5 ("fix frequent -EBUSY at rmdir") adds a busy wait loop to
> the rmdir. It looks like what I am seeing here and indicates that some
> cgroup subsystem is busy, indefinitely.
>
The commit had caused a bug about rmdir, but it was fixed by the commit 88703267.
The fix was merged in 2.6.31, so it seems that you hit a new one...

> I have not worked out how to reproduce it quickly. My only way is to
> complete a 'dd' command in the cgroup, but then the problem is so rare it
> is slow progress.
>
> Documentation/cgroup.memory.txt describes how force_empty can be required
> in some cases. Does this mean that with the patch above, these cases will
> now spin on rmdir(), instead of returning -EBUSY? How can produce a
> reliable test case requiring memory.force_empty to be used, to test this?
>
You don't need to touch "force_empty". rmdir() does what "force_empty" does.

> Or is it likely to be some other cause, and how best to find it?
>
What cgroup subsystem did you mount where the directory existed you tried
to rmdir() first ?
If you mounted several subsystems on the same hierarchy, can you mount them
separately to narrow down the cause ?


Thanks,
Daisuke Nishimura.


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