Messages in this thread | | | Date | Fri, 7 Jan 2011 16:22:07 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC][PATCH 17/18] sched: Move the second half of ttwu() to the remote cpu |
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On 01/06, Peter Zijlstra wrote: > > On Wed, 2011-01-05 at 22:07 +0100, Oleg Nesterov wrote: > > > I'll try to read it once again with the fresh head, though ;) > > I also have a couple of very minor nits... In particular, perhaps > > TASK_WAKING can die... > > I think it might.. I'll do a patch at the end removing it, lets see what > happens.
Yes, ttwu can just set TASK_RUNNING. But, otoh, perhaps the special state makes sense anyway, say, it can help to debug the problems. We can even have TASK_WAKING_CONTRIBUTES_TO_LOAD insetad of ->sched_contributes_to_load. But this all is very minor.
A couple of questions...
Why sched_fork() does set_task_cpu() ? Just curious, it seems that wake_up_new_task() does all we need.
ttwu_queue_remote() does "struct task_struct *next = NULL". Probably "next = rq->wake_list" makes more sens. Otherwise the first cmpxchg() always fails if rq->wake_list != NULL.
Doesn't __migrate_task() need pi_lock? Consider:
1. A task T runs on CPU_0, it does set_current_state(TASK_INTERRUBTIBLE)
2. some CPU does set_cpus_allowed_ptr(T, new_mask), new_mask doesn't include CPU_0.
T is running, cpumask_any_and() picks CPU_1, set_cpus_allowed_ptr() drops pi_lock and rq->lock before stop_one_cpu().
3. T calls schedule() and becomes deactivated.
4. CPU_2 does try_to_wake_up(T, TASK_INTERRUPTIBLE), takes pi_lock and sees on_rq == F.
5. set_cpus_allowed_ptr() resumes and calls stop_one_cpu(cpu => 1).
6. cpu_stopper_thread() runs on CPU_1 and calls ____migrate_task(). It locks CPU_0 and CPU_1 rq's and checks task_cpu() == src_cpu.
7. CPU_2 calls select_task_rq(), it returns (to simplify) 2.
Now try_to_wake_up() does set_task_cpu(T, 2), and calls ttwu_queue()->ttwu_do_activate()->activate_task().
8. __migrate_task() on CPU_1 sees p->on_rq and starts the deactivate/activate dance racing with ttwu_do_activate() on CPU_2.
And a final question. This is really, really minor, but activate_task/deactivate_task are not symmetric, the former always sets p->on_rq. Looks correct, but imho a bit confusing and can complicate the understanding. Since p->on_rq is cleared explicitly by schedule(), perhaps it can be set explicitly to in try_to_wake_up_*. Or, perhaps, activate/deactivate can check ENQUEUE_WAKEUP/DEQUEUE_SLEEP and set/clear p->on_rq. Once again, this is purely cosmetic issue.
Oleg.
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