Messages in this thread Patch in this message |  | | | Date | Wed, 19 May 2010 17:43:56 +0530 | | From | "Amit K. Arora" <> | | Subject | [PATCH v2] Make sure timers have migrated before killing migration_thread |
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On Wed, May 19, 2010 at 11:31:55AM +0200, Peter Zijlstra wrote: > On Wed, 2010-05-19 at 14:35 +0530, Amit K. Arora wrote:
Hi Peter,
Thanks for the review!
> > diff -Nuarp linux-2.6.34.org/kernel/sched.c linux-2.6.34/kernel/sched.c > > --- linux-2.6.34.org/kernel/sched.c 2010-05-18 22:56:21.000000000 -0700 > > +++ linux-2.6.34/kernel/sched.c 2010-05-18 22:58:31.000000000 -0700 > > @@ -5942,14 +5942,26 @@ migration_call(struct notifier_block *nf > > cpu_rq(cpu)->migration_thread = NULL; > > break; > > > > + case CPU_POST_DEAD: > > + /* > > + Bring the migration thread down in CPU_POST_DEAD event, > > + since the timers should have got migrated by now and thus > > + we should not see a deadlock between trying to kill the > > + migration thread and the sched_rt_period_timer. > > + */ > > Faulty comment style that, please use: > > /* > * text > * goes > * here > */
Sure.
> > + cpuset_lock(); > > + rq = cpu_rq(cpu); > > + kthread_stop(rq->migration_thread); > > + put_task_struct(rq->migration_thread); > > + rq->migration_thread = NULL; > > + cpuset_unlock(); > > + break; > > + > > The other problem is more urgent though, CPU_POST_DEAD runs outside of > the hotplug lock and thus the above becomes a race where we could > possible kill off the migration thread of a newly brought up cpu: > > cpu0 - down 2 > cpu1 - up 2 (allocs a new migration thread, and leaks the old one) > cpu0 - post_down 2 - frees the migration thread -- oops!
Ok. So, how about adding a check in CPU_UP_PREPARE event handling too ? The cpuset_lock will synchronize, and thus avoid race between killing of migration_thread in up_prepare and post_dead events.
Here is the updated patch. If you don't like this one too, do you mind suggesting an alternate approach to tackle the problem ? Thanks !
-- Regards, Amit Arora
Signed-off-by: Amit Arora <aarora@in.ibm.com> Signed-off-by: Gautham R Shenoy <ego@in.ibm.com> -- diff -Nuarp linux-2.6.34.org/kernel/sched.c linux-2.6.34/kernel/sched.c --- linux-2.6.34.org/kernel/sched.c 2010-05-18 22:56:21.000000000 -0700 +++ linux-2.6.34/kernel/sched.c 2010-05-19 04:47:49.000000000 -0700 @@ -5900,6 +5900,19 @@ migration_call(struct notifier_block *nf case CPU_UP_PREPARE: case CPU_UP_PREPARE_FROZEN: + cpuset_lock(); + rq = cpu_rq(cpu); + /* + * Since we now kill migration_thread in CPU_POST_DEAD event, + * there may be a race here. So, lets cleanup the old + * migration_thread on the rq, if any. + */ + if (unlikely(rq->migration_thread)) { + kthread_stop(rq->migration_thread); + put_task_struct(rq->migration_thread); + rq->migration_thread = NULL; + } + cpuset_unlock(); p = kthread_create(migration_thread, hcpu, "migration/%d", cpu); if (IS_ERR(p)) return NOTIFY_BAD; @@ -5942,14 +5955,34 @@ migration_call(struct notifier_block *nf cpu_rq(cpu)->migration_thread = NULL; break; + case CPU_POST_DEAD: + /* + * Bring the migration thread down in CPU_POST_DEAD event, + * since the timers should have got migrated by now and thus + * we should not see a deadlock between trying to kill the + * migration thread and the sched_rt_period_timer. + */ + cpuset_lock(); + rq = cpu_rq(cpu); + if (likely(rq->migration_thread)) { + /* + * Its possible that this CPU was onlined (from a + * different CPU) before we reached here and + * migration_thread was cleaned-up in the + * CPU_UP_PREPARE event handling. + */ + kthread_stop(rq->migration_thread); + put_task_struct(rq->migration_thread); + rq->migration_thread = NULL; + } + cpuset_unlock(); + break; + case CPU_DEAD: case CPU_DEAD_FROZEN: cpuset_lock(); /* around calls to cpuset_cpus_allowed_lock() */ migrate_live_tasks(cpu); rq = cpu_rq(cpu); - kthread_stop(rq->migration_thread); - put_task_struct(rq->migration_thread); - rq->migration_thread = NULL; /* Idle task back to normal (off runqueue, low prio) */ raw_spin_lock_irq(&rq->lock); update_rq_clock(rq);
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