linux-kernel - Re: timer code oops when calling mod_delayed

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Date:	Mon, 2 Nov 2015 09:48:41 -1000
From:	Chris Worley <chris.worley@...marydata.com>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	bfields@...ldses.org,
	Michael Skralivetsky <michael.skralivetsky@...marydata.com>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: timer code oops when calling mod_delayed_work

On Sat, Oct 31, 2015 at 11:54 AM, Jeff Layton <jlayton@...chiereds.net> wrote:
> On Sat, 31 Oct 2015 17:31:07 -0400
> Tejun Heo <tj@...nel.org> wrote:
...
>>
>> > I have asked Chris and Michael to see if they can bisect it down, but
>> > it may be a bit before they can get that done. Any insight you might
>> > have in the meantime would helpful.
>>
>> Yeah, I'd love to find out how reproducible the issue is.  If the
>> problem is rarely reproducible, it might make sense to try
>> instrumentation before trying bisection as it *could* be a latent bug
>> which has been there all along and bisecting to the commit introducing
>> the code wouldn't help us too much.
>>
>
> It seems fairly reproducible, at least on v4.3-rc7 kernels:
>
> This came about when I asked them to perf test some nfsd patches that I
> have queued up. I patched a Fedora 4.3-rc7 kernel and wanted to see
> what the perf delta was (with NFSv3, fwiw):
>
>     Patched kernels here: http://koji.fedoraproject.org/koji/taskinfo?taskID=11598089
>
>     Unpatched kernels here: http://koji.fedoraproject.org/koji/buildinfo?buildID=694377
>
> Michael was using the SPEC SFS VDI workload to test, and was able to
> get the same panic on both kernels. So it does seem to be reproducible.
> It might even be possible to tune the VM to make the shrinker fire more
> often, which may help tickle this more.
>
> In any case, I've asked them to try something v4.2-ish and see if it's
> reproducible there, and then try v4.1 if it is. I figure anything
> earlier is probably not worth testing if it still fails on v4.1. If it
> turns out not to be reproducible on those earlier kernels then we can
> bisect from there to track it down.

The trick seems to be the NFS thread count: I initially though this
was SFS/VDI specific, but when I ratcheted up the thread count to what
Michael was using, 256 threads oopses on fio (throughput) benchmarks
too.

In bisecting kernels, it appeared between 4.2.3-200 and 4.2.5-200 (all
the 4.2.4 kernels were bad).

Jeff has a lead on this...

Chris
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