Messages in this thread | | | Date | Mon, 07 Dec 2009 13:34:27 +0900 | From | Tejun Heo <> | Subject | Re: [PATCH 4/7] sched: implement force_cpus_allowed() |
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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.
http://thread.gmane.org/gmane.linux.kernel/896268/focus=896294
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.
Thanks.
-- tejun
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