linux-kernel - Re: Scheduler regression from caffcdd8d27ba78730d5540396ce72ad022aff2c

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Message-ID: <20140718141648.GE20603@laptop.programming.kicks-ass.net>
Date:	Fri, 18 Jul 2014 16:16:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Bruno Wolff III <bruno@...ff.to>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Josh Boyer <jwboyer@...hat.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Scheduler regression from
 caffcdd8d27ba78730d5540396ce72ad022aff2c

On Fri, Jul 18, 2014 at 08:01:26AM -0500, Bruno Wolff III wrote:
> build_sched_domain: cpu: 0 level: SMT cpu_map: 0-3 tl->mask: 0,2
> [    0.254433] build_sched_domain: cpu: 0 level: MC cpu_map: 0-3 tl->mask: 0
> [    0.254516] build_sched_domain: cpu: 0 level: DIE cpu_map: 0-3 tl->mask: 0-3
> [    0.254600] build_sched_domain: cpu: 1 level: SMT cpu_map: 0-3 tl->mask: 1,3
> [    0.254683] build_sched_domain: cpu: 1 level: MC cpu_map: 0-3 tl->mask: 1
> [    0.254766] build_sched_domain: cpu: 1 level: DIE cpu_map: 0-3 tl->mask: 0-3
> [    0.254850] build_sched_domain: cpu: 2 level: SMT cpu_map: 0-3 tl->mask: 0,2
> [    0.254932] build_sched_domain: cpu: 2 level: MC cpu_map: 0-3 tl->mask: 2
> [    0.255005] build_sched_domain: cpu: 2 level: DIE cpu_map: 0-3 tl->mask: 0-3
> [    0.255091] build_sched_domain: cpu: 3 level: SMT cpu_map: 0-3 tl->mask: 1,3
> [    0.255176] build_sched_domain: cpu: 3 level: MC cpu_map: 0-3 tl->mask: 3
> [    0.255260] build_sched_domain: cpu: 3 level: DIE cpu_map: 0-3 tl->mask: 0-3

*blink*...

That's, shall we say, unexpected. Let me ponder that a bit. HPA any clue
why a machine might report such a weird topology? AFAIK threads _always_
share cache.  So how can cpu_coregroup_mask be a subset (instead of a
superset) of topology_thread_cpumask?

Let me go stare at the x86 topology mask setup code.
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