linux-kernel - Re: [PATCH 3/3] sched: Implement interface for cgroup unified hierarchy

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Date:	Thu, 17 Sep 2015 11:52:45 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>, lizefan@...wei.com,
	cgroups <cgroups@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-team <kernel-team@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] sched: Implement interface for cgroup unified
 hierarchy

Hello,

On Thu, Sep 17, 2015 at 05:10:49PM +0200, Peter Zijlstra wrote:
> Subject: sched: Refuse to unplug a CPU if this will violate user task affinity
> 
> Its bad policy to allow unplugging a CPU for which a user set explicit
> affinity, either strictly on this CPU or in case this was the last
> online CPU in its mask.
> 
> Either would end up forcing the thread on a random other CPU, violating
> the sys_sched_setaffinity() constraint.

Shouldn't this at least handle suspend differently?  Otherwise any
userland task would be able to block suspend.

> Disallow this by default; root might not be aware of all user
> affinities, but can negotiate and change affinities for all tasks.
>
> Provide a sysctl to go back to the old behaviour.

I don't think a sysctl is a good way to control this as that breaks
the invariant - all tasks always have some cpus online in its affinity
mask - which otherwise can be guaranteed.

If we wanna go this way, let's plesae start the discussion in a
separate thread with detailed explanation on implications of the
change.

Thanks.

-- 
tejun
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