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SubjectRe: [PATCH 4/7] sched: implement force_cpus_allowed()
Hello, Peter.

On 12/04/2009 07:43 PM, Peter Zijlstra wrote:
>>> force_cpus_allowed() will be used for concurrency-managed workqueue.
>> Would still like to know why all this is needed.
> That is, what problem do these new-fangled workqueues have and why is
> this a good solution.

This is the original RFC posting of cmwq which includes the whole
thing. I'm a few days away from posting a new version but the usage
of force_cpus_allowed() remains the same.

There are two tests which are bypassed by the force_ variant.

* PF_THREAD_BOUND. This is used to mark tasks which are bound to a
cpu using kthread_bind() to be bound permanently. However, new
trustee based workqueue hotplugging decouples per-cpu workqueue
flushing with cpu hot plug/unplugging. This is necessary because
with cmwq, long running works can be served by regular workqueues,
so delaying completion of hot plug/unplugging till certain works are
flushed isn't feasible. So, what becomes necessary is the ability
to re-bind tasks which has PF_THREAD_BOUND set but unbound from its
now offline cpu which is coming online again.

* cpu_active() test. CPU activeness encloses cpu online status which
is the actual on/offline state. Workqueues need to keep running
while a cpu is going down and with cmwq keeping workqueues running
involves creation of new workers (it consists part of
forward-progress guarantee and one of cpu down callbacks might end
up waiting on completion of certain works).

The problem with active state is that during cpu down, active going
off doesn't mean all tasks have been migrated off the cpu, so
without a migration interface which is synchronized with the actual
offline migration, it is difficult to guarantee that all works are
either running on the designated cpu if the cpu is online or all
work on other cpus if the cpu is offline.

Another related problem is that there's no way to monitor the cpu
activeness change notifications.



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