From: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
mingo@kernel.org, pjt@google.com, paul@paulmenage.org,
akpm@linux-foundation.org, rjw@sisk.pl, nacc@us.ibm.com,
paulmck@linux.vnet.ibm.com, tglx@linutronix.de,
seto.hidetoshi@jp.fujitsu.com, rob@landley.net, tj@kernel.org,
mschmidt@redhat.com, berrange@redhat.com,
nikunj@linux.vnet.ibm.com, vatsa@linux.vnet.ibm.com,
linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
linux-pm@vger.kernel.org
Subject: Re: [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets handling upon CPU hotplug
Date: Fri, 4 May 2012 14:57:47 -0700 [thread overview]
Message-ID: <20120504215747.GE3054@linux.vnet.ibm.com> (raw)
In-Reply-To: <1336167265.6509.83.camel@twins>
On 04.05.2012 [23:34:25 +0200], Peter Zijlstra wrote:
> On Fri, 2012-05-04 at 14:27 -0700, Nishanth Aravamudan wrote:
> > > - if you retain it for cpuset but not others that's confusing (too);
> >
> > That's a good point.
> >
> > Related, possibly counter-example, and perhaps I'm wrong about it. When
> > we hot-unplug a CPU, and a task's scheduler affinity (via
> > sched_setaffinity) refers to that CPU only, do we kill that task? Can
> > you sched_setaffinity a task to a CPU that is offline (alone or in a
> > group of possible CPUs)? Or is it allowed to run anywhere? Do we destroy
> > its affinity policy when that situation is run across?
>
> See a few emails back, we destroy the affinity. Current cpuset behaviour
> can be said to match that.
Ah you're right, sorry for glossing over that case. Does that also
happen if you affinitize it to a group of CPUs?
Seems not, we "remember" the original mask in that case:
# taskset -p f $$
pid 1424's current affinity mask: ff
pid 1424's new affinity mask: f
# grep Cpus_allowed /proc/self/status
Cpus_allowed: 0000000f
Cpus_allowed_list: 0-3
# echo 0 > /sys/devices/system/cpu/cpu2/online
# grep Cpus_allowed /proc/self/status
Cpus_allowed: 0000000f
Cpus_allowed_list: 0-3
So ... it seems like we come to a crossroads of sorts? I would think
cpusets and sched_setaffinity should behave the same in terms of
hotplug.
*Maybe* a compromise is that we remember cpuset information up to the
empty cpuset, once you empty a cpuset, you forget everything? That
roughly corresponds to your and my test-case results?
Maybe that's more work than it's worth. It seems like, though, they
should have some similarity in functionality.
> > Or do we restore the task to the CPU again when we re-plug it?
>
> Nope that information is lost forever from the kernels pov.
>
> Keeping this information around for the off-chance of needing it is
> rather expensive (512 bytes per task for your regular distro kernel that
> has NR_CPUS=4096).
Yep, that's another good point.
Thanks,
Nish
--
Nishanth Aravamudan <nacc@us.ibm.com>
IBM Linux Technology Center
next prev parent reply other threads:[~2012-05-04 21:58 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-04 19:17 [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets handling upon CPU hotplug Srivatsa S. Bhat
2012-05-04 19:17 ` [PATCH v2 1/7] cpusets, hotplug: Implement cpuset tree traversal in a helper function Srivatsa S. Bhat
2012-05-04 19:18 ` [PATCH v2 2/7] cpusets, hotplug: Restructure functions that are invoked during hotplug Srivatsa S. Bhat
2012-05-04 19:18 ` [PATCH v2 3/7] cpusets: Introduce 'user_cpus_allowed' and rework the semantics of 'cpus_allowed' Srivatsa S. Bhat
2012-05-04 19:19 ` [PATCH v2 4/7] CPU hotplug, cpusets: Workout hotplug handling for cpusets Srivatsa S. Bhat
2012-05-04 19:19 ` [PATCH v2 5/7] Docs, cpusets: Update the cpuset documentation Srivatsa S. Bhat
2012-05-04 22:28 ` Rob Landley
2012-05-04 19:20 ` [PATCH v2 6/7] cpusets: Optimize the implementation of guarantee_online_cpus() Srivatsa S. Bhat
2012-05-04 19:20 ` [PATCH v2 7/7] cpusets: Remove out-dated comment about cpuset_track_online_cpus Srivatsa S. Bhat
2012-05-04 19:24 ` [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets handling upon CPU hotplug Peter Zijlstra
2012-05-04 19:58 ` Srivatsa S. Bhat
2012-05-04 20:14 ` Peter Zijlstra
2012-05-04 20:28 ` Peter Zijlstra
2012-05-04 20:49 ` Nishanth Aravamudan
2012-05-04 21:01 ` Peter Zijlstra
2012-05-04 21:27 ` Nishanth Aravamudan
2012-05-04 21:32 ` Peter Zijlstra
2012-05-04 21:34 ` Peter Zijlstra
2012-05-04 21:57 ` Nishanth Aravamudan [this message]
2012-05-04 21:38 ` Peter Zijlstra
2012-05-04 20:46 ` Nishanth Aravamudan
2012-05-04 20:56 ` Peter Zijlstra
2012-05-04 21:30 ` Nishanth Aravamudan
2012-05-04 21:44 ` Peter Zijlstra
2012-05-05 15:24 ` Alan Stern
2012-05-05 17:44 ` Paul E. McKenney
2012-05-05 18:56 ` Rafael J. Wysocki
2012-05-08 13:07 ` Daniel P. Berrange
2012-05-05 4:39 ` Mike Galbraith
2012-05-05 17:15 ` Srivatsa S. Bhat
2012-05-07 15:26 ` Jiang Liu
2012-05-09 9:12 ` Srivatsa S. Bhat
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