Re: timer code oops when calling mod_delayed_work

[Jeff Layton]

On Mon, 2 Nov 2015 14:56:33 -0500
Jeff Layton <jlayton@xxxxxxxxxxxxxxx> wrote:

> On Mon, 2 Nov 2015 09:48:41 -1000
> Chris Worley <chris.worley@xxxxxxxxxxxxxxx> wrote:
> 
> > On Sat, Oct 31, 2015 at 11:54 AM, Jeff Layton <jlayton@xxxxxxxxxxxxxxx> wrote:
> > > On Sat, 31 Oct 2015 17:31:07 -0400
> > > Tejun Heo <tj@xxxxxxxxxx> 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
> 
> Thanks Chris. This commit went in between those kernels:
> 
> commit a7c571f2e3ff9243ba41c242486f53dbca37d864
> Author: Shaohua Li <shli@xxxxxx>
> Date:   Wed Sep 30 09:05:30 2015 -0700
> 
>     workqueue: make sure delayed work run in local cpu
>     
>     commit 874bbfe600a660cba9c776b3957b1ce393151b76 upstream.
>     
> 
> ...and I suspect that it's the culprit. That commit causes this code to
> always use add_timer_on, which seems to have different semantics from
> add_timer.
> 
> I'm going to build a v4.2.5 kernel with that patch reverted to confirm
> it, but it seems likely...
> 

(dropping Lai's address, and adding Shaohua Li's)

Ok, I built a kernel with that patch reverted and that seems to fix the
problem.

Looking at the patch, I guess the main difference is that we're no
longer using add_timer for unbound workqueue tasks. That said, we
should have possession of the PENDING bit before calling add_timer_on.

So...I'm a little stumped. Any thoughts as to where the race could be?
-- 
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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