linux-kernel - Re: High context switch rate, ksoftirqd's chewing cpu

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Date:	Fri, 30 Nov 2012 14:52:00 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Zlatko Calusic <zlatko.calusic@...on.hr>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: High context switch rate, ksoftirqd's chewing cpu

Hello, Zlatko.

Sorry about the delay.  Your message was in my spam folder.  The
attachment seems to have confused the filter.

On Sat, Nov 17, 2012 at 02:01:29PM +0100, Zlatko Calusic wrote:
> This week I spent some hours tracking a regression in 3.7 kernel
> that was producing high context switch rate on one of my machines. I
> carefully bisected between 3.6 and 3.7-rc1 and eventually found this
> commit a culprit:
> 
> commit e7c2f967445dd2041f0f8e3179cca22bb8bb7f79
> Author: Tejun Heo <tj@...nel.org>
> Date:   Tue Aug 21 13:18:24 2012 -0700
> 
>     workqueue: use mod_delayed_work() instead of __cancel + queue
...
> 
> Then I carefully reverted chunk by chunk to find out what exact
> change is responsible for the regression. You can find it attached
> as wq.patch (to preserve whitespace). Very simple modification with
> wildly different behavior on only one of my machines, weird. I'm
> also attaching ctxt/s graph that shows the impact nicely. I'll
> gladly provide any additional info that could help you resolve this.
> 
> Please Cc: on reply (not subscribed to lkml).
> 
> Regards,
> -- 
> Zlatko

> diff --git a/block/blk-core.c b/block/blk-core.c
> index 4b4dbdf..4b8b606 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -319,10 +319,8 @@ EXPORT_SYMBOL(__blk_run_queue);
>   */
>  void blk_run_queue_async(struct request_queue *q)
>  {
> -	if (likely(!blk_queue_stopped(q))) {
> -		__cancel_delayed_work(&q->delay_work);
> -		queue_delayed_work(kblockd_workqueue, &q->delay_work, 0);
> -	}
> +	if (likely(!blk_queue_stopped(q)))
> +		mod_delayed_work(kblockd_workqueue, &q->delay_work, 0);
>  }
>  EXPORT_SYMBOL(blk_run_queue_async);

That's intersting.  Is there anything else noticeably different than
the ctxsw counts?  e.g. CPU usage, IO throughput / latency, etc...
Also, can you please post the kernel boot log from the machine?  I
assume that the issue is readily reproducible?  Are you up for trying
some debug patches?

Thanks.

-- 
tejun
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