linux-kernel - Re: [PATCH 1/3] sched: select_task_rq() should check cpu_active() like select_fallback

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Message-ID: <20151012121657.GP3816@twins.programming.kicks-ass.net>
Date:	Mon, 12 Oct 2015 14:16:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	heiko.carstens@...ibm.com, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched: select_task_rq() should check cpu_active()
 like select_fallback_rq()

On Sat, Oct 10, 2015 at 08:53:09PM +0200, Oleg Nesterov wrote:
> I do not understand the cpu_active() check in select_fallback_rq().
> x86 doesn't need it, and the recent commit dd9d3843755d "sched: Fix
> cpu_active_mask/cpu_online_mask race" documents the fact that on any
> architecture we can ignore !active starting from CPU_ONLINE stage.
> 
> But any possible reason why do we need this check in "fallback" must
> equally apply to select_task_rq().

So the reason, from vague memory, is that we want to allow per-cpu
threads to start/stop before/after active.

active 'should' really only govern load-balancer bits or so.
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