linux-kernel - Re: [PATCH v2] sched: wake-affine throttle

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Date:	Wed, 22 May 2013 17:25:56 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
	Alex Shi <alex.shi@...el.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Turner <pjt@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Ram Pai <linuxram@...ibm.com>
Subject: Re: [PATCH v2] sched: wake-affine throttle

On 05/22/2013 04:49 PM, Peter Zijlstra wrote:
[snip]
>>
>> CC: Ingo Molnar <mingo@...nel.org>
>> CC: Peter Zijlstra <peterz@...radead.org>
>> CC: Mike Galbraith <efault@....de>
>> CC: Alex Shi <alex.shi@...el.com>
>> Suggested-by: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Michael Wang <wangyun@...ux.vnet.ibm.com>

Thanks for your reply, I've looking forward it for a long time...

> 
> So I utterly hate this patch. I hate it worse than your initial buddy
> patch :/

Then we nuke it, and figure out the better one ;-)

> 
> And I know its got a Suggested-by there; but that was when you led me to
> believe that wake_affine() itself was expensive to run; its not, its the
> result of those runs you don't like.

Both are the reason, it's just the game between gain & lost & cost, your
suggestion definitely is a good choice, otherwise I won't pay time on
it, and I will call it's the best one if we are searching for a quick fix.

> 
> While we have a ton (too many to be sure) scheduler tunables, users
> shouldn't ever need to actually touch those. Its just that every time we
> have to make a random choice its as easy to make it a debug knob as to
> hardcode it.
> 
> The problem with this patch is that users _have_ to frob knobs and while
> doing so potentially wreck other workloads.
> 
> To make it worse, the knob isn't anything fundamental, its a random
> hack.

So we discard.

> 
> So I would really either improve the smarts of wake_affine, with for
> example your wake buddy relation thing (and simply exempt [Soft]IRQs) or
> kill wake_affine and be done with it.

No kill...we show mercy, I will back to the wakeup-buddy and let's
forgot the IRQ case temporarily unless some regression report appear.

> 
> Either avenue has the risk of regressing some workload, but at least
> when that happens (and people report it) we'll have a counter-example to
> learn from and incorporate.

I've not test the hackbench with wakeup-buddy before, will do it this
time, I suppose the 15% illegal income will suffered, anyway, it's
illegal :)

Regards,
Michael Wang

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