linux-kernel - Re: [PATCH] cfq-iosched: Fix the incorrect timeslice accounting with forced

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Message-ID: <20100409072915.GF10103@kernel.dk>
Date:	Fri, 9 Apr 2010 09:29:16 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Divyesh Shah <dpshah@...gle.com>
Cc:	vgoyal@...hat.com, linux-kernel@...r.kernel.org, nauman@...gle.com,
	ctalbott@...gle.com
Subject: Re: [PATCH] cfq-iosched: Fix the incorrect timeslice accounting
	with  forced_dispatch.

On Thu, Apr 08 2010, Divyesh Shah wrote:
> When CFQ dispatches requests forcefully due to a barrier or changing iosched,
> it runs through all cfqq's dispatching requests and then expires each queue.
> However, it does not activate a cfqq before flushing its IOs resulting in
> using stale values for computing slice_used.
> This patch fixes it by calling activate queue before flushing reuqests from
> each queue.
> 
> This is useful mostly for barrier requests because when the iosched is changing
> it really doesnt matter if we have incorrect accounting since we're going to
> break down all structures anyway.
> 
> We also now expire the current timeslice before moving on with the dispatch
> to accurately account slice used for that cfqq.

Good catch, applied for 2.6.34.

-- 
Jens Axboe

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