Messages in this thread Patch in this message |  | | Date | Mon, 4 Jul 2016 14:21:43 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH RFC] sched: Make wake_up_nohz_cpu() handle CPUs going offline |
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On Fri, Jul 01, 2016 at 05:15:06PM -0700, Paul E. McKenney wrote: > On Sat, Jul 02, 2016 at 01:49:56AM +0200, Frederic Weisbecker wrote: > > On Fri, Jul 01, 2016 at 11:40:54AM -0700, Paul E. McKenney wrote: > > > On Fri, Jul 01, 2016 at 01:29:59AM +0200, Frederic Weisbecker wrote: > > > > > +/* > > > > > + * Wake up the specified CPU. If the CPU is going offline, it is the > > > > > + * caller's responsibility to deal with the lost wakeup, for example, > > > > > + * by hooking into the CPU_DEAD notifier like timers and hrtimers do. > > > > > + */ > > > > > void wake_up_nohz_cpu(int cpu) > > > > > { > > > > > - if (!wake_up_full_nohz_cpu(cpu)) > > > > > + if (cpu_online(cpu) && !wake_up_full_nohz_cpu(cpu)) > > > > > > > > So at this point, as we passed CPU_DYING, I believe the CPU isn't visible in the domains > > > > anymore (correct me if I'm wrong), therefore get_nohz_timer_target() can't return it, > > > > unless smp_processor_id() is the only alternative. > > > > > > Right, but the timers have been posted long before even CPU_UP_PREPARE. > > > From what I can see, they are left alone until CPU_DEAD. Which means > > > that if you try to mod_timer() them between CPU_DYING and CPU_DEAD, > > > you can get the above splat. > > > > > > Or am I missing somthing subtle here? > > > > Yes that's exactly what I meant. It happens on mod_timer() calls > > between CPU_DYING and CPU_DEAD. I just wanted to clarify the > > conditions for it to happen: the fact that it shouldn't concern > > remote CPU targets, only local pinned timers. > > OK. What happens in the following sequence of events? > > o CPU 5 posts a timer, which might well be locally pinned. > This is rcu_torture_reader() posting its on-stack timer > creatively named "t". > > o CPU 5 starts going offline, so that rcu_torture_reader() gets > migrated to CPU 6. > > o CPU 5 reaches CPU_DYING but has not yet reached CPU_DEAD. > > o CPU 6 invokes mod_timer() on its timer "t". > > Wouldn't that trigger the scenario that I am seeing?
No I don't think so because "t" is then going to be enqueued to CPU 6. __mod_timer() -> get_target_base() uses local accessor on timer base.
So the target of pinned timers is always the CPU of the caller.
> > > > BTW, it seems that rcutorture stops its kthreads after CPU_DYING, is it expected that > > > > it queues timers at this stage? > > > > > > Hmmm... From what I can see, rcutorture cleans up its priority-boost > > > kthreads at CPU_DOWN_PREPARE time. The other threads are allowed to > > > migrate wherever the scheduler wants, give or take the task shuffling. > > > The task shuffling only excludes one CPU at a time, and I have seen > > > this occur when multiple CPUs were running, e.g., 0, 2, and 3 while > > > offlining 1. > > > > But if rcutorture kthreads are cleaned up at CPU_DOWN_PREPARE, they > > shouldn't be calling mod_timer() on CPU_DYING time. Or there are other > > rcutorture threads? > > The rcu_torture_reader() kthreads aren't associated with any particular > CPU, so when CPUs go offline, they just get migrated to other CPUs. > This allows them to execute on those other CPUs between CPU_DYING and > CPU_DEAD time, correct?
Indeed. Timers get migrated on CPU_DEAD only. So if rcu_torture_reader() enqueued a pinned timer to CPU 5, then get migrated to CPU 6, the timer may well fire after CPU_DYING on CPU 5 and even re-enqueue itself in CPU 5, provided the timer is self-enqueued.
> > Other rcutorture kthreads -are- bound to specific CPUs, but they are > testing priority boosting, not simple reading.
I see.
> > > > Besides which, doesn't the scheduler prevent anything but the idle > > > thread from running after CPU_DYING time? > > > > Indeed migrate_tasks() is called on CPU_DYING but pinned kthreads, outside > > smpboot, have their own way to deal with hotplug through notifiers. > > Agreed, but the rcu_torture_reader() kthreads aren't pinned, so they > should migrate automatically at CPU_DYING time.
Yes indeed.
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 7f2cae4620c7..1a91fc733a0f 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -580,6 +580,8 @@ static bool wake_up_full_nohz_cpu(int cpu) > * If needed we can still optimize that later with an > * empty IRQ. > */ > + if (cpu_is_offline(cpu)) > + return true;
Preferably put this under the tick_nohz_full_cpu() below because it has a static key optimizations. Distros build NO_HZ_FULL but don't use it 99.99999% of the time.
> if (tick_nohz_full_cpu(cpu)) { > if (cpu != smp_processor_id() || > tick_nohz_tick_stopped()) > @@ -590,6 +592,11 @@ static bool wake_up_full_nohz_cpu(int cpu) > return false; > } > > +/* > + * Wake up the specified CPU. If the CPU is going offline, it is the > + * caller's responsibility to deal with the lost wakeup, for example, > + * by hooking into the CPU_DEAD notifier like timers and hrtimers do. > + */
I think it's more transparent than that for the caller. migrate_timers() is called soon after and it takes care of waking up the destination of the migration if necessary. So the caller shouldn't care after all.
But the cpu_is_offline() check above may need a comment about that.
Thanks!
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