linux-kernel - Re: [RFC PATCH] sched: wakeup buddy

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Message-ID: <1362035917.4460.105.camel@marge.simpson.net>
Date:	Thu, 28 Feb 2013 08:18:37 +0100
From:	Mike Galbraith <efault@....de>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ram Pai <linuxram@...ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC PATCH] sched: wakeup buddy

On Thu, 2013-02-28 at 14:38 +0800, Michael Wang wrote:

> +				/*
> +				 * current is the only task on rq and it is
> +				 * going to sleep, current cpu will be a nice
> +				 * candidate for p to run on.
> +				 */

The sync hint only means it might be going to sleep soon, and even then,
there can still be enough execution overlap to be a win to schedule
cross core.  Sched pipe numbers will always be much prettier if you do
wakeup cpu affine, as it's ~100% scheduler and ~100% sync.  You may lose
a lot on other stuff if you interpret the hint as gospel truth.

IMHO, sched pipe is a "how fat have I become" benchmark, not "how well
do I perform".  The scheduler performs well when it makes more work
happen.  Playing ping-pong with yourself is _exercise_, not a job :)

-Mike

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