Messages in this thread | | | Date | Fri, 15 May 2020 14:03:46 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 00/13] Reconcile NUMA balancing decisions with the load balancer v6 |
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On Fri, May 15, 2020 at 01:17:32PM +0200, Peter Zijlstra wrote: > On Fri, May 15, 2020 at 09:47:40AM +0100, Mel Gorman wrote: > > > However, the wakeups are so rapid that the wakeup > > happens while the server is descheduling. That forces the waker to spin > > on smp_cond_load_acquire for longer. In this case, it can be cheaper to > > add the task to the rq->wake_list even if that potentially requires an IPI. > > Right, I think Rik ran into that as well at some point. He wanted to > make ->on_cpu do a hand-off, but simply queueing the wakeup on the prev > cpu (which is currently in the middle of schedule()) should be an easier > proposition. > > Maybe something like this untested thing... could explode most mighty, > didn't thing too hard. > > --- > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index fa6c19d38e82..c07b92a0ee5d 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2312,7 +2312,7 @@ static void wake_csd_func(void *info) > sched_ttwu_pending(); > } > > -static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) > +static void __ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) > { > struct rq *rq = cpu_rq(cpu); > > @@ -2354,6 +2354,17 @@ bool cpus_share_cache(int this_cpu, int that_cpu) > { > return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu); > } > + > +static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) > +{ > + if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) { > + sched_clock_cpu(cpu); /* Sync clocks across CPUs */ > + __ttwu_queue_remote(p, cpu, wake_flags); > + return true; > + } > + > + return false; > +} > #endif /* CONFIG_SMP */ > > static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags) > @@ -2362,11 +2373,8 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags) > struct rq_flags rf; > > #if defined(CONFIG_SMP) > - if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) { > - sched_clock_cpu(cpu); /* Sync clocks across CPUs */ > - ttwu_queue_remote(p, cpu, wake_flags); > + if (ttwu_queue_remote(p, cpu, wake_flags)) > return; > - } > #endif > > rq_lock(rq, &rf); > @@ -2550,7 +2558,15 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > if (p->on_rq && ttwu_remote(p, wake_flags)) > goto unlock; > > + if (p->in_iowait) { > + delayacct_blkio_end(p); > + atomic_dec(&task_rq(p)->nr_iowait); > + } > + > #ifdef CONFIG_SMP > + p->sched_contributes_to_load = !!task_contributes_to_load(p); > + p->state = TASK_WAKING; > + > /* > * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be > * possible to, falsely, observe p->on_cpu == 0. > @@ -2581,15 +2597,10 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > * This ensures that tasks getting woken will be fully ordered against > * their previous state and preserve Program Order. > */ > - smp_cond_load_acquire(&p->on_cpu, !VAL); > - > - p->sched_contributes_to_load = !!task_contributes_to_load(p); > - p->state = TASK_WAKING; > + if (READ_ONCE(p->on_cpu) && __ttwu_queue_remote(p, cpu, wake_flags)) > + goto unlock; > > - if (p->in_iowait) { > - delayacct_blkio_end(p); > - atomic_dec(&task_rq(p)->nr_iowait); > - } > + smp_cond_load_acquire(&p->on_cpu, !VAL); > > cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags); > if (task_cpu(p) != cpu) {
I don't see a problem with moving the updating of p->state to the other side of the barrier but I'm relying on the comment that the barrier is only related to on_rq and on_cpu.
However, I'm less sure about what exactly you intended to do. __ttwu_queue_remote is void so maybe you meant to use ttwu_queue_remote. In that case, we potentially avoid spinning on on_rq for wakeups between tasks that do not share CPU but it's not clear why it would be specific to remote tasks. If you meant to call __ttwu_queue_remote unconditionally, it's not clear why that's now safe when smp_cond_load_acquire() cared about on_rq being 0 before queueing a task for wakup or directly waking it up.
Also because __ttwu_queue_remote() now happens before select_task_rq(), is there not a risk that in some cases we end up stacking tasks unnecessarily?
> @@ -2597,14 +2608,6 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > psi_ttwu_dequeue(p); > set_task_cpu(p, cpu); > } > - > -#else /* CONFIG_SMP */ > - > - if (p->in_iowait) { > - delayacct_blkio_end(p); > - atomic_dec(&task_rq(p)->nr_iowait); > - } > - > #endif /* CONFIG_SMP */ > > ttwu_queue(p, cpu, wake_flags);
-- Mel Gorman SUSE Labs
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