Messages in this thread | | | Date | Fri, 6 Jul 2007 10:23:39 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC] Thread Migration Preemption |
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* Oleg Nesterov (oleg@tv-sign.ru) wrote: > Mathieu Desnoyers wrote: > > > > This patch adds the ability to protect critical sections from migration to > > another CPU without disabling preemption. > > > > Typical use: > > > > migration_disable(); > > local_inc(&__get_cpu_var(&my_local_t_var)); > > migration_enable(); > > > > --- linux-2.6-lttng.orig/kernel/sched.c 2007-07-05 16:28:15.000000000 -0400 > > +++ linux-2.6-lttng/kernel/sched.c 2007-07-05 16:53:24.000000000 -0400 > > @@ -1996,6 +1996,7 @@ > > * 1) running (obviously), or > > * 2) cannot be migrated to this CPU due to cpus_allowed, or > > * 3) are cache-hot on their current CPU. > > + * 4) migration preemption is non 0 for this non running task. > > */ > > if (!cpu_isset(this_cpu, p->cpus_allowed)) > > return 0; > > @@ -2003,6 +2004,8 @@ > > > > if (task_running(rq, p)) > > return 0; > > + if (task_thread_info(p)->migration_count) > > + return 0; >
Hi Oleg,
> Question: This means that the task could be preempted, but can't sleep, yes? >
No exactly. This means that the task can be preempted, and can therefore sleep, but we do not allow this sleeping task to be migrated to a different CPU.
> Because try_to_wake_up() can change ->cpu. Shouldn't might_sleep() check > ->migration_count then? Or we can change try_to_wake_up(). >
If we look at try_to_wake_up, it seems to take the new_cpu from task_cpu(), which comes from the thread info ->cpu, updated by set_task_cpu(). The *_move_tasks seems to be responsible for calling can_migrate_task() to see if, in a deferred way, the migration_thread can use set_task_cpu() to change the task's cpu.
move_task_off_dead_cpu() does not seem to respect this though: it could potentially move away a process while it is in a critical section by forcing a __migrate_task. I guess I should put a test in __migrate_task also so move_task_off_dead_cpu() can try to repeat the migration.
> What if the task does copy_process() under migration_disable() ? Child > gets a copy of ->migration_count. >
Right. Neighter does copy_process set the preemption count. It seems to be dealt by sched_fork() when the thread is first scheduled in to make sure that the preempt count is 1. I guess is would be safer to set the migration count to 0 there too. (or set it to one if we ever decide to make it the kernel default).
> Also, cpu_down() still can migrate this task to another CPU. >
Yes, just noted this earlier. I first thought it was using can_migrate_task(), but it does not seem so.
Thanks,
Mathieu
> Oleg. >
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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