Re: Kernel NFSd CPU hog?

[Andreas Heinlein <aheinlein@gmx.com>]

On 02.07.2012 21:07, Jeff Layton wrote:
> On Mon, 02 Jul 2012 11:43:48 +0200
> Andreas Heinlein <aheinlein@gmx.com> wrote:
>
>> Hello,
>>
>> we have a strange NFS problem with a newly setup Linux server, and I
>> hope someone here can help.
>>
>> The symptom is that, slowly over time (speaking of several days up to 2
>> weeks), the kernel nfsd processes/threads consume more and more CPU
>> until the system finally becomes unresponsive. We recorded system
>> activity with sar, which shows that CPU (system) usage slowly rises
>> after reboot from about 1% to nearly 100% over the course of several
>> days. Load averages stay around 0.1-0.3 until 100% are reached, up to
>> this point the problem is almost not noticable from the clients. Then
>> load averages climb up to 30.0; at this point the system becomes more or
>> less unusable and has to be restarted. 'top' output shows the CPU usage
>> evenly distributed across all nfsd threads.
>>
>> The system is a fairly recent, though entry level server with a Core i3
>> and 4G RAM, hosting the home directories for about 15-20 clients. CPU
>> activity does not drop at night, when no clients are connected. It is
>> running Debian 6.0 with linux 3.2.0 (from the backports repository),
>> with nfs-utils 1.2.5 (also from the backports repository). I suspect
>> that these backports might be the culprit, but since we need this kernel
>> for other purposes, and I cannot reboot that machine during office
>> hours, I'd rather not try going back to the official Debian kernel
>> without good reasons. If there are known problems, I'd give it a try.
>>
> Find the pid of one of the nfsd threads that's spinning, then get a
> stack trace from it:
>
>      # cat /proc/<pidofnfsd>/stack
>
> ...that should give us some idea of what it's doing.
>
> --
> Jeff Layton <jlayton@redhat.com>
> --
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Hello,

I've run into the problem again, and did a 'watch cat 
/proc/<pidofnfsd>/stack'. It actually seems to be doing something, 
because the stack trace changes every now and then, but mostly looks like

[<c1038be2>] try_to_wake_up+0x144/0x14d
[<c1045d26>] lock_timer_base+0x19/0x34
[<c10462fd>] __mod_timer+0x10c/0x116
[<c1045d41>] process_timeout+0x0/0x5
[<f858a243>] svc_recv+0x2e2/0x698 [sunrpc]
[<c1038beb>] default_wake_function+0x0/0x8
[<f8640748>] nfsd+0x90/0x108 [nfsd]
[<f86406b8>] nfsd+0x0/0x108 [nfsd]
[<c105176b>] kthread+0x63/0x68
[<c1051708>] kthread+0x0/0x68
[<c12dadbe>] kernel_thread_helper+0x6/0x10
[<ffffffff>] 0xffffffff

Meanwhile, I've found a quite recent thread on this list named "3.0+ NFS 
issues", and within two links to Ubuntu bug reports 
(https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/879334 and 
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1006446) and again 
to a kernel bug (https://bugzilla.kernel.org/show_bug.cgi?id=40912), all 
suggesting that this is indeed a kernel 3.0 problem.

So I will try going back to 2.6.32 and hope this issue gets fixed soon.

Thanks for your help!