linux-kernel - Re: sched: Avoid SMT siblings in select_idle

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Date:	Thu, 17 Nov 2011 02:59:13 +0100
From:	Mike Galbraith <efault@....de>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>
Subject: Re: sched: Avoid SMT siblings in select_idle_sibling() if possible

On Wed, 2011-11-16 at 10:37 -0800, Suresh Siddha wrote:
> On Wed, 2011-11-16 at 01:24 -0800, Mike Galbraith wrote:

> > At SIBLING, group = 0,4 = 0x5, 0x5 & 0xff = 1 = target.
> 
> Mike, At the sibling level, domain span will be 0,4 which is 0x5. But
> there are two individual groups. First group just contains cpu0 and the
> second group contains cpu4.
> 
> So if cpu0 is busy, we will check the next group to see if it is idle
> (which is cpu4 in your example). So we will return cpu-4.

Oh duh, right.

> It should be ok. Isn't it?

Nope, wasn't ok.  I'll double check today though, and bend, spindle
and/or mutilate as required.

	-Mike

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