Messages in this thread | | | From | Chengming Zhou <> | Date | Sun, 20 Nov 2022 10:22:40 +0800 | Subject | Re: [PATCH v3] sched: async unthrottling for cfs bandwidth |
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On 2022/11/19 03:25, Josh Don wrote: > On Fri, Nov 18, 2022 at 4:47 AM Peter Zijlstra <peterz@infradead.org> wrote: >> >> preempt_disable() -- through rq->lock -- also holds off rcu. Strictly >> speaking this here is superfluous. But if you want it as an annotation, >> that's fine I suppose. > > Yep, I purely added this as extra annotation for future readers. > >> Ideally we'd first queue all the remotes and then process local, but >> given how all this is organized that doesn't seem trivial to arrange. >> >> Maybe have this function return false when local and save that cfs_rq in >> a local var to process again later, dunno, that might turn messy. > > Maybe something like this? Apologies for inline diff formatting. > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 012ec9d03811..100dae6023da 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5520,12 +5520,15 @@ static bool distribute_cfs_runtime(struct > cfs_bandwidth *cfs_b) > struct cfs_rq *cfs_rq; > u64 runtime, remaining = 1; > bool throttled = false; > + int this_cpu = smp_processor_id(); > + struct cfs_rq *local_unthrottle = NULL; > + struct rq *rq; > + struct rq_flags rf; > > rcu_read_lock(); > list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq, > throttled_list) { > - struct rq *rq = rq_of(cfs_rq); > - struct rq_flags rf; > + rq = rq_of(cfs_rq); > > if (!remaining) { > throttled = true; > @@ -5556,14 +5559,36 @@ static bool distribute_cfs_runtime(struct > cfs_bandwidth *cfs_b) > cfs_rq->runtime_remaining += runtime; > > /* we check whether we're throttled above */ > - if (cfs_rq->runtime_remaining > 0) > - unthrottle_cfs_rq_async(cfs_rq); > + if (cfs_rq->runtime_remaining > 0) { > + if (cpu_of(rq) != this_cpu || > + SCHED_WARN_ON(local_unthrottle)) { > + unthrottle_cfs_rq_async(cfs_rq); > + } else { > + local_unthrottle = cfs_rq; > + } > + } else { > + throttled = true; > + }
Hello,
I don't get the point why local unthrottle is put after all the remote cpus, since this list is FIFO? (earliest throttled cfs_rq is at the head)
Should we distribute runtime in the FIFO order?
Thanks.
> > next: > rq_unlock_irqrestore(rq, &rf); > } > rcu_read_unlock(); > > + /* > + * We prefer to stage the async unthrottles of all the remote cpus > + * before we do the inline unthrottle locally. Note that > + * unthrottle_cfs_rq_async() on the local cpu is actually synchronous, > + * but it includes extra WARNs to make sure the cfs_rq really is > + * still throttled. > + */ > + if (local_unthrottle) { > + rq = cpu_rq(this_cpu); > + rq_lock_irqsave(rq, &rf); > + unthrottle_cfs_rq_async(local_unthrottle); > + rq_unlock_irqrestore(rq, &rf); > + } > + > return throttled; > } > > Note that one change we definitely want is the extra setting of > throttled = true in the case that cfs_rq->runtime_remaining <= 0, to > catch the case where we run out of runtime to distribute on the last > entity in the list. > >>> + >>> + /* Already enqueued */ >>> + if (SCHED_WARN_ON(!list_empty(&cfs_rq->throttled_csd_list))) >>> + return; >>> + >>> + list_add_tail(&cfs_rq->throttled_csd_list, &rq->cfsb_csd_list); >>> + >>> + smp_call_function_single_async(cpu_of(rq), &rq->cfsb_csd); >> >> Hurmph.. so I was expecting something like: >> >> first = list_empty(&rq->cfsb_csd_list); >> list_add_tail(&cfs_rq->throttled_csd_list, &rq->cfsb_csd_list); >> if (first) >> smp_call_function_single_async(cpu_of(rq), &rq->cfsb_csd); >> >> But I suppose I'm remembering the 'old' version. I don't think it is >> broken as written. There's a very narrow window where you'll end up >> sending a second IPI for naught, but meh. > > The CSD doesn't get unlocked until right before we call the func(). > But you're right that that's a (very) narrow window for an extra IPI. > Please feel free to modify the patch with that diff if you like. > >> >>> +} >> >> Let me go queue this thing, we can always improve upon matters later. > > Thanks! Please add at least the extra assignment of 'throttled = true' > from the diff above, but feel free to squash both the diffs if it > makes sense to you.
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