Re: bcache: Fix a writeback performance regression

[Kent Overstreet <kmo-PEzghdH756F8UrSeD/g0lQ@public.gmane.org>]

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...