linux-kernel - Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

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Message-ID: <1399013469.5233.85.camel@marge.simpson.net>
Date:	Fri, 02 May 2014 08:51:09 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	mingo@...nel.org, peterz@...radead.org,
	george.mccollister@...il.com, ktkhai@...allels.com
Subject: Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

On Fri, 2014-05-02 at 08:36 +0200, Mike Galbraith wrote:

> Reason why is that case was on a box where FAIR_SLEEPERS is disabled by
> default, meaning there is no such thing as wakeup preemption.  Guess
> what happens when you don't have shared LLC for a fast/light wakee to
> escape to when the waker is a pig.  The worst thing possible in that
> case is to wake affine.  Leave the poor thing wherever it was, else it
> will take a latency hit that need not have been.

Oh yeah, and you'll see similar issues playing with kvm.  No escape
routes are available, as no llc domain exists.  Globally do a sync
wakeup CPU affine, and for some loads that will induce massive wreckage
where as If select_isle_sibling() had been there to save the day, all
would have been peachy.  Is it good, or is it evil... depends.

-Mike

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