Messages in this thread | | | Date | Sat, 12 Jul 2008 12:04:36 +0200 | From | "Dmitry Adamushko" <> | Subject | Re: current linux-2.6.git: cpusets completely broken |
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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
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