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

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Date:	Wed, 02 Dec 2009 09:47:59 -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:

> On Wed, Dec 02, 2009 at 03:14:22PM +0100, Corrado Zoccolo wrote:
>> Hi Jeff,
>> On Wed, Dec 2, 2009 at 2:42 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> > Corrado Zoccolo <czoccolo@...il.com> writes:
>> >
>> >> Idling logic was disabled in some corner cases, leading to unfair share
>> >> for noidle queues.
>> >> * the idle timer was not armed if there were other requests in the
>> >>   driver. unfortunately, those requests could come from other workloads,
>> >>   or queues for which we don't enable idling. So we will check only
>> >>   pending requests from the active queue
>> >> * rq_noidle check on no-idle queue could disable the end of tree idle if
>> >>   the last completed request was rq_noidle. Now, we will disable that
>> >>   idle only if all the queues served in the no-idle tree had rq_noidle
>> >>   requests.
>> >>
>> >> Reported-by: Vivek Goyal <vgoyal@...hat.com>
>> >> Signed-off-by: Corrado Zoccolo <czoccolo@...il.com>
>> >
>> >> @@ -2606,17 +2608,27 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
>> >>                       cfq_clear_cfqq_slice_new(cfqq);
>> >>               }
>> >>               /*
>> >> -              * If there are no requests waiting in this queue, and
>> >> -              * there are other queues ready to issue requests, AND
>> >> -              * those other queues are issuing requests within our
>> >> -              * mean seek distance, give them a chance to run instead
>> >> -              * of idling.
>> >> +              * Idling is not enabled on:
>> >> +              * - expired queues
>> >> +              * - idle-priority queues
>> >> +              * - async queues
>> >> +              * - queues with still some requests queued
>> >> +              * - when there is a close cooperator
>> >>                */
>> >
>> > I'm not sure this logic is correct.  Is this for the 2.6.33 branch?
>> Yes.
>> > If so, the coop flag now means that multiple processes share the same
>> > cfqq.  Are you sure this is the right thing to do for close cooperators?
>> I'm not sure. I didn't change the logic for close cooperators:

Heh, right you are.

>> -               else if (cfqq_empty && !cfq_close_cooperator(cfqd, cfqq) &&
>> -                        sync && !rq_noidle(rq))
>> -                       cfq_arm_slice_timer(cfqd);
>> +               else if (sync && cfqq_empty &&
>> +                        !cfq_close_cooperator(cfqd, cfqq)) {
>> +                       cfqd->noidle_tree_requires_idle |= !rq_noidle(rq);
>> 
>> I changed the rq_noidle part, and rewrote the comment to be aligned
>> with the code.
>> So I don't mind if you improve (or just remove) the close cooperator part.
>> Probably, you should do a test where close cooperating processes are competing
>> with a sequential reader, to see the effect of idling or not on them.
>> 
>
> I also can't find what's wrong with this. Previously we were not merging
> close cooperators in a single queue. So if we found a close cooperator
> we chose to not idle and move to that close cooperator. Now we try to
> merge all the close cooperators in a single queue. But that merging has
> not taken place yet and will happen when next request comes. 

The coop flag is not set until the merge has taken place.

> A normal sequential reader will not find the close cooperator. Only the
> queues which should be merged will find the close cooperator. If anyway
> these queues are going to be merged soon, there is proably no point in
> continuing to idle on this queue in case we found a close cooperator.
>
> So, to me even in new code by jeff, it probably is fine to continue with
> policy of not idling if we found a close cooperator.

That would mean changing the check from cfqq_coop to cfqq->new_queue !=
NULL.

Cheers,
Jeff
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