On 12/04/21 14:03, Peter Zijlstra wrote: > On Thu, Mar 11, 2021 at 03:13:04PM +0000, Valentin Schneider wrote: >> Peter Zijlstra <pet...@infradead.org> writes: >> > @@ -7910,6 +7908,14 @@ int sched_cpu_deactivate(unsigned int cp >> > } >> > rq_unlock_irqrestore(rq, &rf); >> > >> > + /* >> > + * From this point forward, this CPU will refuse to run any task that >> > + * is not: migrate_disable() or KTHREAD_IS_PER_CPU, and will actively >> > + * push those tasks away until this gets cleared, see >> > + * sched_cpu_dying(). >> > + */ >> > + balance_push_set(cpu, true); >> > + >> >> AIUI with cpu_dying_mask being flipped before even entering >> sched_cpu_deactivate(), we don't need this to be before the >> synchronize_rcu() anymore; is there more than that to why you're punting it >> back this side of it? > > I think it does does need to be like this, we need to clearly separate > the active=true and balance_push_set(). If we were to somehow observe > both balance_push_set() and active==false, we'd be in trouble. >
I'm afraid I don't follow; we're replacing a read of rq->balance_push with cpu_dying(), and those are still written on the same side of the synchronize_rcu(). What am I missing? >> > #ifdef CONFIG_SCHED_SMT >> > /* >> > * When going down, decrement the number of cores with SMT present. >> >> > @@ -8206,7 +8212,7 @@ void __init sched_init(void) >> > rq->sd = NULL; >> > rq->rd = NULL; >> > rq->cpu_capacity = rq->cpu_capacity_orig = SCHED_CAPACITY_SCALE; >> > - rq->balance_callback = NULL; >> > + rq->balance_callback = &balance_push_callback; >> > rq->active_balance = 0; >> > rq->next_balance = jiffies; >> > rq->push_cpu = 0; >> > @@ -8253,6 +8259,7 @@ void __init sched_init(void) >> > >> > #ifdef CONFIG_SMP >> > idle_thread_set_boot_cpu(); >> > + balance_push_set(smp_processor_id(), false); >> > #endif >> > init_sched_fair_class(); >> > >> >> I don't get what these two changes do - the end result is the same as >> before, no? > > Not quite; we have to make sure the state of the offline CPUs matches > that of a CPU that's been offlined. For consistency if nothing else, but > it's actually important for a point below. > >> Also, AIUI this patch covers the cpu_dying -> !cpu_dying rollback case >> since balance_push gets numbed down by !cpu_dying. What about the other way >> around (hot-plug failure + rollback)? We may have allowed !pcpu tasks on the >> now-dying CPU, and we'd need to re-install the balance_push callback. > > This is in fact handled. Note how the previous point initialized the > offline CPU to have the push_callback installed. > > Also note how balance_push() re-instates itself unconditionally. > > So the thing is, we install the push callback on deactivate() and leave > it in place until activate, it is always there, regardless what way > we're moving. > > However, it is only effective whild going down, see the early exit. Oooh, I can't read, only the boot CPU gets its callback uninstalled in sched_init()! So secondaries keep push_callback installed up until sched_cpu_activate(), but as you said it's not effective unless a rollback happens. Now, doesn't that mean we should *not* uninstall the callback in sched_cpu_dying()? AFAIK it's possible for the initial secondary CPU boot to go fine, but the next offline+online cycle fails while going up - that would need to rollback with push_callback installed.
