linux-kernel - Re: [patch 4/4] sched: Distangle worker accounting from rq->lock

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Date:	Thu, 23 Jun 2011 13:35:07 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Jens Axboe <axboe@...nel.dk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 4/4] sched: Distangle worker accounting from rq->lock

Hello, Ingo.

On Thu, Jun 23, 2011 at 12:44 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Tejun Heo <tj@...nel.org> wrote:
>
>> The patch description is simply untrue.  It does affect how wq
>> behaves under heavy CPU load.  The effect might be perfectly okay
>> but more likely it will result in subtle suboptimal behaviors under
>> certain load situations which would be difficult to characterize
>> and track down.  Again, the trade off (mostly killing of
>> ttwu_local) could be okay but you can't get away with just claiming
>> "there's no harm".
>
> Well, either it can be measured or not. If you can suggest a specific
> testing method to Thomas, please do.

Crafting a test case where the suggested change results in worse
behavior isn't difficult (it ends up waking/creating workers which it
doesn't have to between ttwu and actual execution); however, as with
any micro benchmark, the problem is with assessing whether and how
much it would matter in actual workloads (whatever that means) and
workqueue is used throughout the kernel with widely varying patterns
making drawing conclusion a bit tricky.  Given that, changing the
behavior for the worse just for this cleanup doesn't sound like too
sweet a deal.  Is there any other reason to change the behavior
(latency, whatever) other than the ttuw_local ugliness?

Thanks.

-- 
tejun
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