linux-kernel - Re: bcache: Fix a writeback performance regression

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Message-ID: <20130821211035.GB4657@kmo-pixel>
Date:	Wed, 21 Aug 2013 14:10:35 -0700
From:	Kent Overstreet <kmo@...erainc.com>
To:	Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-bcache@...r.kernel.org
Subject: Re: bcache: Fix a writeback performance regression

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.

*swears*

I just saw that this morning, but I assumed it was a bug in my stripe
aware scan code that isn't in this branch yet - thanks for letting me
know that's not the case.

I shall work on a fix for the fix...
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