Messages in this thread | | | Date | Tue, 22 Jun 2021 14:51:41 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 2/2] sched: Plug race between SCA, hotplug and migration_cpu_stop() |
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Hi Valentin,
I've been looking at this on and off and I'm afraid I'm still not convinced, almost certainly due to my own ignorance, but hey :)
On Wed, Jun 02, 2021 at 08:43:31PM +0100, Valentin Schneider wrote: > On 02/06/21 16:26, Will Deacon wrote: > > On Tue, Jun 01, 2021 at 05:59:56PM +0100, Valentin Schneider wrote: > >> On 26/05/21 21:57, Valentin Schneider wrote: > >> > + dest_cpu = arg->dest_cpu; > >> > + if (task_on_rq_queued(p)) { > >> > + /* > >> > + * A hotplug operation could have happened between > >> > + * set_cpus_allowed_ptr() and here, making dest_cpu no > >> > + * longer allowed. > >> > + */ > >> > + if (!is_cpu_allowed(p, dest_cpu)) > >> > + dest_cpu = select_fallback_rq(cpu_of(rq), p); > >> > + /* > >> > + * dest_cpu can be victim of hotplug between is_cpu_allowed() > >> > + * and here. However, per the synchronize_rcu() in > >> > + * sched_cpu_deactivate(), it can't have gone lower than > >> > + * CPUHP_AP_ACTIVE, so it's safe to punt it over and let > >> > + * balance_push() route it elsewhere. > >> > + */ > >> > + update_rq_clock(rq); > >> > + rq = move_queued_task(rq, &rf, p, dest_cpu); > >> > >> So, while digesting this I started having doubts vs pcpu kthreads since > >> they're allowed on online CPUs. The bogus scenario here would be picking a > >> !active && online CPU, and see it go !online before the move_queued_task(). > >> > >> Now, to transition from online -> !online, we have to go through > >> take_cpu_down() which is issued via a stop_machine() call. This means the > >> transition can't happen until all online CPUs are running the stopper task > >> and reach MULTI_STOP_RUN. > >> > >> migration_cpu_stop() being already a stopper callback should thus make it > >> "atomic" vs takedown_cpu(), meaning the above should be fine. > > > > I'd be more inclined to agree with your reasoning if migration_cpu_stop() > > couldn't itself call stop_one_cpu_nowait() to queue more work for the > > stopper thread. What guarantees that takedown_cpu() can't queue its stopper > > work in the middle of that? > > > > Harumph... > > So something like all CPUs but one are running their take_cpu_down() > callback because one is still running migration_cpu_stop(), i.e.: > > CPU0 CPU1 ... CPUN > <stopper> <stopper> <stopper> > migration_cpu_stop() take_cpu_down()@MULTI_STOP_PREPARE take_cpu_down()@MULTI_STOP_PREPARE > > If CPU0 hits that else if (pending) condition, it'll queue a > migration_cpu_stop() elsewhere (say CPU1), then run the take_cpu_down() > callback which follows in its work->list. > > If the CPU being brought down is anything else than CPU1, it shouldn't > really matter. If it *is* CPU1, then I think we've got some guarantees. > > Namely, there's no (read: there shouldn't be any) way for a task to > still be on CPU1 at this point; per sched_cpu_wait_empty(), > migration-disabled tasks and pcpu kthreads must vacate the CPU before it > then (migrate_disable regions must be bounded, and pcpu kthreads are > expected to be moved away by their respective owners).
I agree with that, but the stopper is still running on CPU1 and so migration_cpu_stop() could still queue work there after sched_cpu_wait_empty() has returned but before stop_machine_park(), afaict.
Actually, it looks like migration_cpu_stop() ignores the return value of stop_one_cpu_nowait(), so if the stopper thread has been parked I think we'll quietly do nothing there as well.
> With the above, I don't really see how migration_cpu_stop() could queue > itself again on an about-to-die CPU (thus racing with the take_cpu_down() > callback), and even if it did, then we'd still run the callback before > parking the stopper thread (no surprise callback persisting during > offline), in which case we'd either see the task as having moved somewhere > sensible, or we'll queue yet another callback.
In this case, I think we'd run the migration_cpu_stop() callback, but then __migrate_task() would fail to move the task because the CPU would be !online.
Will
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