linux-kernel - Re: block CFQ: avoid moving request to different queue

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Message-ID: <5098F6E1.6010502@kernel.dk>
Date:	Tue, 06 Nov 2012 12:39:13 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Shaohua Li <shli@...nel.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: block CFQ: avoid moving request to different queue

On 2012-11-06 12:34, Shaohua Li wrote:
> request is queued in cfqq->fifo list. Looks it's possible we are
> moving a request from one cfqq to another in request merge case. In
> such case, adjusting the fifo list order doesn't make sense and is
> impossible if we don't iterate the whole fifo list.
> 
> My test does hit one case the two cfqq are different, but didn't cause
> kernel crash, maybe it's because fifo list isn't used frequently.
> Anyway, from the code logic, this is buggy.

Good find!! Usually we never merge between cfqq's as our lookup basis is
the cfqq. And yes, the fifo generally isn't used a lot, it's only a
fallback measure to prevent inter-cfqq unfairness.

Applied to for-3.8/core.

And lets re-enable the recursive merging, please do send a patch for
that too.

-- 
Jens Axboe

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