Re: bcache: Fix a writeback performance regression

[Stefan Priebe]

Am 22.08.2013 01:47, schrieb Kent Overstreet:
> On Tue, Aug 20, 2013 at 10:07:45AM +0200, Stefan Priebe - Profihost AG wrote:
> > Am 20.08.2013 10:01, schrieb Stefan Priebe - Profihost AG:
> > > Am 20.08.2013 00:27, schrieb Kent Overstreet:
> > > > On Mon, Aug 19, 2013 at 12:09:24AM +0200, Stefan Priebe wrote:
> > > > > Vanilla 3.10.7 + bcache: Fix a writeback performance regression
> > > > >
> > > > > http://pastebin.com/raw.php?i=LXZk4cMH
> > > >
> > > > Whoops, at first I thought this was the same bug as one I'd already been
> > > > chasing down that had been a harmless bug - turns out I didn't look
> > > > closely enough at the backtrace.
> > > >
> > > > What happened is background writeback is deadlocking, because for some
> > > > reason the workqueue it's running out of is a singlethreaded workqueue,
> > > > so as soon as it decides to queue enough writeback bios that it has to
> > > > sleep on that semaphore (which often won't happen due to the PD
> > > > controller based ratelimiting) - boom, deadlock.
> > > >
> > > > Here's the fixup patch I just tested and am applying:
> >
> > Oh i'm now seeing very high CPU spikes of kworker... i don't see if i
> > remove bcache: Fix a writeback performance regression.
>
> Are you able to reproduce it? I'm not having any luck reproducing it...

Hi,

yes but only on production system it seems it happens when there is a 
big bunch in dirty and having heavy 4K random I/O.

Stefan
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