linux-kernel - Re: [PATCH 1/2]block cfq: make queue preempt work for queues from different workload

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Date:	Tue, 11 Jan 2011 22:07:47 +0100
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH 1/2]block cfq: make queue preempt work for queues from
 different workload

Hi Shaohua,
On Tue, Jan 11, 2011 at 9:51 AM, Shaohua Li <shaohua.li@...el.com> wrote:
> I got this:
>             fio-874   [007]  2157.724514:   8,32   m   N cfq874 preempt
>             fio-874   [007]  2157.724519:   8,32   m   N cfq830 slice expired t=1
>             fio-874   [007]  2157.724520:   8,32   m   N cfq830 sl_used=1 disp=0 charge=1 iops=0 sect=0
>             fio-874   [007]  2157.724521:   8,32   m   N cfq830 set_active wl_prio:0 wl_type:0
>             fio-874   [007]  2157.724522:   8,32   m   N cfq830 Not idling. st->count:1
> cfq830 is an async queue, and preempted by a sync queue cfq874. But since we
> have cfqg->saved_workload_slice mechanism, the preempt is a nop.
> Looks currently our preempt is totally broken if the two queues are not from
> the same workload type.
> Below patch fixes it. This will might make async queue starvation, but it's
> what our old code does before cgroup is added.
have you measured latency improvements by un-breaking preemption?
AFAIK, preemption behaviour changed since 2.6.33, before cgroups were
added, and the latency before the changes that weakened preemption in
2.6.33 was far worse.

Thanks,
Corrado
>
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>
>
> ---
>  block/cfq-iosched.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
>
> Index: linux/block/cfq-iosched.c
> ===================================================================
> --- linux.orig/block/cfq-iosched.c      2011-01-11 16:24:56.000000000 +0800
> +++ linux/block/cfq-iosched.c   2011-01-11 16:41:49.000000000 +0800
> @@ -3284,10 +3284,19 @@ cfq_should_preempt(struct cfq_data *cfqd
>  */
>  static void cfq_preempt_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
>  {
> +       struct cfq_queue *old_cfqq = cfqd->active_queue;
> +
>        cfq_log_cfqq(cfqd, cfqq, "preempt");
>        cfq_slice_expired(cfqd, 1);
>
>        /*
> +        * workload type is changed, don't save slice, otherwise preempt
> +        * doesn't happen
> +        */
> +       if (cfqq_type(old_cfqq) != cfqq_type(cfqq))
> +               cfqq->cfqg->saved_workload_slice = 0;
> +
> +       /*
>         * Put the new queue at the front of the of the current list,
>         * so we know that it will be selected next.
>         */
>
>
>
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