linux-kernel - Re: Q: select_fallback_rq() && cpuset

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Message-ID: <20100311152201.GA13888@redhat.com>
Date:	Thu, 11 Mar 2010 16:22:01 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Lai Jiangshan <laijs@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: Q: select_fallback_rq() && cpuset_lock()

On 03/11, Oleg Nesterov wrote:
>
> How can we fix this later? Perhaps we can change
> cpuset_track_online_cpus(CPU_DEAD) to scan all affected cpusets and
> fixup the tasks with the wrong ->cpus_allowed == cpu_possible_mask.

Wait. We need to fix the CPU_DEAD case anyway?

Hmm. 6ad4c18884e864cf4c77f9074d3d1816063f99cd
"sched: Fix balance vs hotplug race" did s/CPU_DEAD/CPU_DOWN_PREPARE/
in cpuset_track_online_cpus(). This doesn't look exactly right to me,
we shouldn't do remove_tasks_in_empty_cpuset() at CPU_DOWN_PREPARE
stage, it can fail.

Otoh. This means that move_task_of_dead_cpu() can never see the
task without active cpus in ->cpus_allowed, it is called later by
CPU_DEAD. So, cpuset_lock() is not needed at all.

Oleg.

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