linux-kernel - Re: [PATCH] task containersv11 add tasks file interface fix for cpusets

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Message-ID: <6599ad830710031110r1a74eafej56f20cc33e72ac81@mail.gmail.com>
Date:	Wed, 3 Oct 2007 11:10:58 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Paul Jackson" <pj@....com>
Cc:	akpm@...ux-foundation.org, nickpiggin@...oo.com.au,
	a.p.zijlstra@...llo.nl, serue@...ibm.com, clg@...ibm.com,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com,
	rientjes@...gle.com, svaidy@...ux.vnet.ibm.com, xemul@...nvz.org,
	containers@...ts.osdl.org, balbir@...ux.vnet.ibm.com
Subject: Re: [PATCH] task containersv11 add tasks file interface fix for cpusets

On 10/3/07, Paul Jackson <pj@....com> wrote:
>
> Yes, something in user space has to do it.  It's part of the
> kernel-user cpuset API.  If you change a cpuset's 'cpus', then
> you have to rewrite each pid in its 'tasks' file back to that
> 'tasks' file in order to get that 'cpus' change to be applied
> to the task struct cpus_allowed of each task, and thereby visible
> to the scheduler.

What's the rationale for this?

Given that later in cpusets.txt you say:

>If hotplug functionality is used
>to remove all the CPUs that are currently assigned to a cpuset,
>then the kernel will automatically update the cpus_allowed of all
>tasks attached to CPUs in that cpuset to allow all CPUs

why can't the same thing be done when changing the 'cpus' file manually.

What's wrong with, in update_cpumask(), doing a loop across all
members of the cgroup and updating their cpus_allowed fields?

The existing cpusets API is broken, since a new child can always be
forked between reading the tasks file and doing the writes.

Paul
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