linux-kernel - Re: [PATCH 4/4] cfq-iosched: fix corner cases in idling logic

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Message-ID: <x49tyw9if3m.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 02 Dec 2009 10:13:17 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 4/4] cfq-iosched: fix corner cases in idling logic

Vivek Goyal <vgoyal@...hat.com> writes:

> Does it make a big difference. cfq_close_cooperator() does not seem to be
> relying on coop flag. It will return us a queue if it thinks there is a
> close cooperator. (Irrespective of the fact whether cfqq->new_cfqq has bee
> setup yet or not). IIUC, cfqq->new_cfqq will be set in select_queue(). So
> in case select_queue() has not run yet, then cfqq->new_cfqq = NULL but we
> have a close cooperator.
>
> But I guess this condition will not hit many a times as select_queue()
> happens very frequently on NCQ hardware and the moment select queue finds
> close cooperator it will expire the current queue and above check will not
> even get a chance to turn.
>
> So IIUC, if we are here cfqq->new_cfqq is always NULL otherwise select_queue()
> by now must have expired us and we will not be here. So either we can
> completely remove the check or we can just continue with above check.

Wow, I need another cup of coffee for sure.  You can just ignore me as I
completely misread the code being changed.

Sorry for the noise.  I'll go make some coffee.

Cheers,
Jeff
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