linux-kernel - Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq

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Date:	Fri, 22 Feb 2013 10:57:36 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>, alex.shi@...el.com,
	Ram Pai <linuxram@...ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()

On Fri, 2013-02-22 at 17:11 +0800, Michael Wang wrote:

> Ok, it do looks like wake_affine() lost it's value...

I'm not sure we can say that on this one benchmark, there's a
preemption advantage to running on a single cpu for pipe-test as well.
We'd need to create a better benchmark to test this, one that has some
actual data payload and control over the initial spread of the tasks or
so.

> > Now as far as I can see there's two options, either we find there's
> > absolutely no benefit in wake_affine() as it stands today and we simply
> > disable/remove it, or we go fix it. What we don't do is completely
> > wreck it at atrocious cost.
> 
> I get your point, we should replace wake_affine() with some feature
> which could really achieve the goal to make client and server on same cpu.
> 
> But is the logical that the waker/wakee are server/client(or reversed)
> still works now? that sounds a little arbitrary to me...

Ah, its never really been about server/client per-se. Its just a
specific example -- one that breaks down with the 1:n pgbench
situation.

Wakeups in general can be considered to be a relation, suppose a
hardware interrupt that received some data from a device and issues a
wakeup to a task to consume this data. What CPU would be better suited
to process this data then the one where its already cache hot.

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