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SubjectRe: [PATCH UPDATED 2/4] sched: adjust when cpu_active and cpuset configurations are updated during cpu on/offlining
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On Wed, 2010-06-02 at 18:03 +0200, Tejun Heo wrote:
> Currently, when a cpu goes down, cpu_active is cleared before
> CPU_DOWN_PREPARE starts and cpuset configuration is updated from a
> default priority cpu notifier. When a cpu is coming up, it's set
> before CPU_ONLINE but cpuset configuration again is updated from the
> same cpu notifier.
>
> For cpu notifiers, this presents an inconsistent state. Threads which
> a CPU_DOWN_PREPARE notifier expects to be bound to the CPU can be
> migrated to other cpus because the cpu is no more inactive.
>
> Fix it by updating cpu_active in the highest priority cpu notifier and
> cpuset configuration in the second highest when a cpu is coming up.
> Down path is updated similarly. This guarantees that all other cpu
> notifiers see consistent cpu_active and cpuset configuration.
>
> cpuset_track_online_cpus() notifier is converted to
> cpuset_update_active_cpus() which just updates the configuration and
> now called from cpuset_cpu_[in]active() notifiers registered from
> sched_init_smp(). If cpuset is disabled, cpuset_update_active_cpus()
> degenerates into partition_sched_domains() making separate notifier
> for !CONFIG_CPUSETS unnecessary.
>
> This problem is triggered by cmwq. During CPU_DOWN_PREPARE, hotplug
> callback creates a kthread and kthread_bind()s it to the target cpu,
> and the thread is expected to run on that cpu.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Paul Menage <menage@google.com>
> ---
> Okay, how about this one? Using notifiers seems better for the
> following reasons.
>
> * Rollback on failure.
>
> * cpuset/sched_domain don't expect to be called before smp
> configuration is complete. If hardcoded into cpu_up/down(),
> condition checks need to be added so that they're skipped if the
> system is bringing up the cpus for the first time.
>
> Works fine here w/ CPUSET enabled and disabled.

OK, this looks good.

Thanks Tejun.


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