Messages in this thread | | | Date | Mon, 16 Nov 2015 13:53:51 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and updating to be called once |
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On Wed, Nov 11, 2015 at 08:51:47AM +0900, Byungchul Park wrote: > On Tue, Nov 10, 2015 at 01:16:47PM +0100, Peter Zijlstra wrote: > > So the problem is that as soon as that ->cpu store comes through, the > > other rq->lock can happen, even though we might still hold a rq->lock > > thinking we're serialized. > > > > Take for instance move_queued_tasks(), it does: > > > > dequeue_task(rq, p, 0); > > p->on_rq = TASK_ON_RQ_MIGRATING; > > set_task_cpu(p, new_cpu) { > > __set_task_cpu(); > > > > ^^^ here holding rq->lock is insufficient and the below: > > > > p->sched_class->migrate_task_rq() > > Thank you for explaning in detail, but this's why i asked you.
> Yes, rq->lock is insufficient in this place as you said, but > should migrate_task_rq() be serialized by rq->lock? I might have > agreed with you if the migrate_task_rq() should be serialized by > rq->lock, but I think it's not the case. I think it would be of > if task->pi_lock can work correcly within *if statement* in > set_task_cpu(). Wrong?
So currently, set_task_cpu() is serialized by:
- p->pi_lock; on wakeup - rq->lock; otherwise
(see the #ifdef CONFIG_LOCKDEP comment in set_task_cpu())
This means that sched_class::migrate_task() cannot indeed rely on rq->lock for full serialization, however it still means that task_rq_lock() will fully serialize against the thing.
By changing this, it no longer will.
Even without that; I think such a change, if correct, is very fragile and prone to creating problems later on, and sets bad precedent.
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