linux-kernel - Re: current linux-2.6.git: cpusets completely broken

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Message-ID: <19f34abd0807130150y581abe07j29765a9424cdb02@mail.gmail.com>
Date:	Sun, 13 Jul 2008 10:50:12 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>, "Paul Menage" <menage@...gle.com>,
	"Max Krasnyansky" <maxk@...lcomm.com>, "Paul Jackson" <pj@....com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>, miaox@...fujitsu.com,
	rostedt@...dmis.org, "Thomas Gleixner" <tglx@...utronix.de>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: current linux-2.6.git: cpusets completely broken

On Sun, Jul 13, 2008 at 2:10 AM, Dmitry Adamushko
<dmitry.adamushko@...il.com> wrote:
> Subject: fix cpuset_handle_cpuhp()
>
> The following commit
>
> ---
> commit f18f982abf183e91f435990d337164c7a43d1e6d
> Author: Max Krasnyansky <maxk@...lcomm.com>
> Date:   Thu May 29 11:17:01 2008 -0700
>
> sched: CPU hotplug events must not destroy scheduler domains created by
> the cpusets
> ---
>
> [ Note, with this commit arch_update_cpu_topology is not called any more for CPUSETS. But it's just a nop.
> The whole scheme should be probably reworked later. ]
>
>
> introduced a hotplug-related problem as described below:
>
> [ Basically the fix below just emulates the 'old' behavior of update_sched_domains().
> We call rebuild_sched_domains() for the same hotplug-events as it was called (and is still called
> for !CPUSETS case) in update_sched_domains(). ]
>
>
> Upon CPU_DOWN_PREPARE, update_sched_domains() -> detach_destroy_domains(&cpu_online_map)
> does the following:
>
> /*
>  * Force a reinitialization of the sched domains hierarchy. The domains
>  * and groups cannot be updated in place without racing with the
> balancing
>  * code, so we temporarily attach all running cpus to the NULL domain
>  * which will prevent rebalancing while the sched domains are
> recalculated.
>  */
>
> The sched-domains should be rebuilt when a CPU_DOWN ops. has been
> completed, effectively either upon CPU_DEAD{_FROZEN} (upon success) or
> CPU_DOWN_FAILED{_FROZEN} (upon failure -- restore the things to their
> initial state). That's what update_sched_domains() also does but only
> for !CPUSETS case.
>
> With Max's patch, sched-domains' reinitialization is delegated to
> CPUSETS code:
>
> cpuset_handle_cpuhp() -> common_cpu_mem_hotplug_unplug() ->
> rebuild_sched_domains()
>
> Being called for CPU_UP_PREPARE and if its callback is called after
> update_sched_domains()), it just negates all the work done by
> update_sched_domains() -- i.e. a soon-to-be-offline cpu is included in
> the sched-domains and that makes it visible for the load-balancer
> while the CPU_DOWN ops. is in progress.
>
> __migrate_live_tasks() moves the tasks off a 'dead' cpu (it's already
> "offline" when this function is called).
>
> try_to_wake_up() is called for one of these tasks from another CPU ->
> the load-balancer (wake_idle()) picks up a "dead" CPU and places the
> task on it. Then e.g. BUG_ON(rq->nr_running) detects this a bit later
> -> oops.
>
>
> Signed-off-by: Dmitry Adamushko <dmitry.adamushko@...il.com>

Tested-by: Vegard Nossum <vegard.nossum@...il.com>

Works :-)


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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