linux-kernel - Re: [PATCH] block: strip out locking optimization in put_io

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Message-ID: <4F30C96F.1000905@kernel.dk>
Date:	Tue, 07 Feb 2012 07:49:19 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	Shaohua Li <vivek.goyal2008@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>, mroos@...ux.ee
Subject: Re: [PATCH] block: strip out locking optimization in put_io_context()

On 02/06/2012 10:54 PM, Tejun Heo wrote:
> put_io_context() performed a complex trylock dancing to avoid
> deferring ioc release to workqueue.  It was also broken on UP because
> trylock was always assumed to succeed which resulted in unbalanced
> preemption count.
> 
> While there are ways to fix the UP breakage, even the most
> pathological microbench (forced ioc allocation and tight fork/exit
> loop) fails to show any appreciable performance benefit of the
> optimization.  Strip it out.  If there turns out to be workloads which
> are affected by this change, simpler optimization from the discussion
> thread can be applied later.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> LKML-Reference: <1328514611.21268.66.camel@...10-conroe>
> ---
> I couldn't find any statiscally meaningful advantage of the
> optimization with tight fork/exit tests w/ forced ioc creation on
> fork, which gotta be the most pathological test case for the code
> path.  So, let's remove the ugly optimization.  If I missed sth, we
> can resurrect the simpler optimization later.  Jens, this is on top of
> linus#master without Shaohua's patch.

OK, then I'm fine with cleaning it up. Applied, thanks Tejun.

-- 
Jens Axboe

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