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

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Message-ID: <521323D1.1070503@profihost.ag>
Date:	Tue, 20 Aug 2013 10:07:45 +0200
From:	Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>
To:	Kent Overstreet <kmo@...erainc.com>
CC:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-bcache@...r.kernel.org
Subject: Re: bcache: Fix a writeback performance regression

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.

Stefan
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