linux-kernel - Re: cpusets not cpu hotplug aware

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Message-Id: <20060821221437.255808fa.pj@sgi.com>
Date:	Mon, 21 Aug 2006 22:14:37 -0700
From:	Paul Jackson <pj@....com>
To:	Nathan Lynch <ntl@...ox.com>
Cc:	akpm@...l.org, anton@...ba.org, simon.derr@...l.net,
	nathanl@...tin.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: cpusets not cpu hotplug aware

Nathan wrote:
> I think it would be more sensible for the default (i.e. user hasn't
> explicitly configured any cpusets) behavior on a CONFIG_CPUSETS=y
> kernel to match the behavior with a CONFIG_CPUSETS=n kernel. 

Basically, in this situation, that would mean that the code that
masks a requested sched_setaffinity cpumask with the tasks
cpuset_cpus_allowed():

	cpus_allowed = cpuset_cpus_allowed(p);
	cpus_and(new_mask, new_mask, cpus_allowed);

would not be executed in the case of a kernel configured for cpusets,
when running on a system that wasn't using cpusets.

That's quite doable, and for systems not actually using cpusets, makes
good sense.

But it makes a bit of discontinuity in the system behaviour when
someone starts using cpusets.  If someone makes a cpuset, then suddenly
tasks in the top cpuset start seeing failed sched_setaffinity calls
for any CPU that was brought online after system boot.

If there is some decent way I can get the cpus_allowed of the top
cpuset to track the cpu_online_map, then we avoid this discontinuity
in system behaviour.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.925.600.0401
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