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

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Date:	Tue, 7 Feb 2012 08:47:35 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jens Axboe <axboe@...nel.dk>, 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()

Hello,

On Tue, Feb 07, 2012 at 08:33:15AM -0800, Linus Torvalds wrote:
> Yeah, please just get rid of the crazy code. Maybe *that* fixes the
> regression too, who knows?
> 
> For all we know, the "fast case" is what causes extra locking only to
> then fail and not even be a fast-path.

Yeah, I was about to ask Shaohua to test the version w/o optimization.
With heavily loaded request_queue, trylock failure could be frequent,
which I wasn't testing.

Shaohua, can you please test the version w/o optimization?  Also, can
you please give a bit more details on the setup?  Are there multiple
swap devices?  Is it SSD or rotating disk?

> I think our default action should always be to simplify and clean up
> code, unless you have seriously hard numbers to show that the code
> complexity is worth it.

Sure, it was originally all in put_io_context() and while moving
things to wq, it got progressively complex and I lost sense of
complexity from staring at it too long.

Thanks.

-- 
tejun
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