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From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
	paul@paulmenage.org, rjw@sisk.pl, tj@kernel.org,
	frank.rowand@am.sony.com, pjt@google.com, tglx@linutronix.de,
	lizf@cn.fujitsu.com, prashanth@linux.vnet.ibm.com,
	paulmck@linux.vnet.ibm.com, vatsa@linux.vnet.ibm.com,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>
Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets
Date: Sat, 11 Feb 2012 14:39:29 +0100	[thread overview]
Message-ID: <20120211133929.GB28098@elte.hu> (raw)
In-Reply-To: <1328889127.25989.14.camel@laptop>


* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Thu, 2012-02-09 at 16:11 +0100, Ingo Molnar wrote:
> 
> > > My understanding of the code is that when a CPU is taken 
> > > offline, it is removed from all the cpusets and then the 
> > > scan_for_empty_cpusets() function is run to move tasks from 
> > > empty cpusets to their parent cpusets.
> > 
> > Why is that done that way? offlining a CPU should be an 
> > invariant as far as cpusets are concerned.
> 
> Can't, tasks need to run someplace. There's two choices, add a 
> still online cpu to the now empty cpuset or move the tasks to 
> a parent that still has online cpus.
> 
> Both are destructive.

You aren't thinking hard enough ;-) There's several solutions 
off the top of my mind:

1) refuse the "impossible" offlining of the CPU, with a clear 
   enough error to make it actionable

2) offer a 'forced' offlinign of a CPU that will SIGTERM all 
   tasks that are on the now offline CPU and can only be there.

3) offer a 'nice' offlining variant that moves all orphan tasks
   to their or any other well-defined fallback CPU.

4) *allow* 'impossible' cpusets and just run them on CPU#0 or 
   any other natural approximation. Don't touch the cpuset!

All of these would be exception mechanisms with no need to do 
anything at hot-replug time.

Thanks,

	Ingo

  parent reply	other threads:[~2012-02-11 13:40 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-02-07 18:55 [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets Srivatsa S. Bhat
2012-02-07 18:56 ` [PATCH 1/4] CPU hotplug, cpuset: Maintain a copy of the cpus_allowed mask before CPU hotplug Srivatsa S. Bhat
2012-02-07 18:56 ` [PATCH 2/4] cpuset: Split up update_cpumask() so that its functionality can be reused Srivatsa S. Bhat
2012-02-07 18:57 ` [PATCH 3/4] cpuset: Add function to introduce CPUs to cpusets during CPU online Srivatsa S. Bhat
2012-02-07 18:57 ` [PATCH 4/4] CPU hotplug, cpusets: Differentiate the CPU online and CPU offline callbacks Srivatsa S. Bhat
2012-02-08  3:22 ` [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets Peter Zijlstra
2012-02-08  6:33   ` Srivatsa S. Bhat
2012-02-09  7:57     ` Ingo Molnar
2012-02-09  8:42       ` Srivatsa S. Bhat
2012-02-09 15:11         ` Ingo Molnar
2012-02-10 15:52           ` Peter Zijlstra
2012-02-10 16:53             ` Paul E. McKenney
2012-02-10 17:34               ` Peter Zijlstra
2012-02-10 21:51                 ` Alan Stern
2012-02-10 22:39                   ` Rafael J. Wysocki
2012-02-11  2:07                     ` Peter Zijlstra
2012-02-11  4:26                       ` Srivatsa S. Bhat
2012-02-13 17:47                         ` Srivatsa S. Bhat
2012-02-17 12:15                           ` Srivatsa S. Bhat
2012-02-20 12:49                             ` Peter Zijlstra
2012-02-20 12:59                               ` Srivatsa S. Bhat
2012-02-23  9:57                                 ` Srivatsa S. Bhat
2012-02-24 23:24                                   ` Rafael J. Wysocki
2012-02-27 10:18                                   ` Peter Zijlstra
2012-02-27 12:09                                   ` [tip:sched/urgent] CPU hotplug, cpusets, suspend: Don' t touch cpusets during suspend/resume tip-bot for Srivatsa S. Bhat
2012-02-11 16:00                 ` [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets Paul E. McKenney
2012-02-13 17:47               ` Srivatsa S. Bhat
2012-02-13 20:39                 ` Paul E. McKenney
2012-02-13 20:49                   ` Srivatsa S. Bhat
2012-02-11 13:39             ` Ingo Molnar [this message]
2012-02-10 15:53         ` Peter Zijlstra
2012-02-09 16:43     ` Peter Zijlstra

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