linux-kernel - Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE

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Message-ID: <20150528102127.GD3644@twins.programming.kicks-ass.net>
Date:	Thu, 28 May 2015 12:21:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Josef Bacik <jbacik@...com>
Cc:	riel@...hat.com, mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for
 BALANCE_WAKE

On Wed, May 27, 2015 at 05:22:16PM -0400, Josef Bacik wrote:
> 
> SD_BALANCE_WAKE is supposed to find us an idle cpu to run on, however

sd->flags's SD_BALANCE_WAKE, not sd_flags.

And note that SD_BALANCE_WAKE is not set on domains by default.

> it is just looking for an idle sibling, preferring affinity over all
> else.

Not sure what you're saying here, what affinity?

> This is not helpful in all cases, and SD_BALANCE_WAKE's job is
> to find us an idle cpu, not garuntee affinity.

Your argument is going backwards, SD_BALANCE_WAKE is not actually set.

> Fix this by first
> trying to find an idle sibling, and then if the cpu is not idle fall
> through to the logic to find an idle cpu.  With this patch we get
> slightly better performance than with our forward port of
> SD_WAKE_IDLE. 

This is broken. You most certainly do not want to go do that whole load
balance pass on wakeups. It should be controlled by sd->flags. It is far
too expensive to consider turning that on by default.

In fact, select_idle_sibling() is already too expensive on current
server hardware (far too damn many cpus in a LLC domain).

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