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

["Dmitry Adamushko" <dmitry.adamushko@gmail.com>]

2008/7/12 Linus Torvalds <torvalds@linux-foundation.org>:
>
>
> On Sat, 12 Jul 2008, Vegard Nossum wrote:
>>
>> Can somebody else please test/ack/review it too? This should eventually
>> go into 2.6.26 if it doesn't break anything else.
>
> And Dmitry, _please_ also explain what was going on. Why did things break
> from calling common_cpu_mem_hotplug_unplug() too much? That function is
> called pretty randomly anyway (for just about any random CPU event), so
> why did it fail in some circumstances?

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. is completed,
effectivelly 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()

which as you've said "called pretty randomly anyway", e.g. for CPU_UP_PREPARE.

[ ah, then rebuild_sched_domains() should not be there. It should be
nop for MEMPLUG events I presume - should make another patch. ]

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.

Now another funny thing is that we probably have a memory leak with
common_cpu_mem_hotplug_unplug() "randomly" calling
rebuild_sched_domains() and sometimes re-allocating domains when they
already exist.


>
>                Linus
>

-- 
Best regards,
Dmitry Adamushko