Re: [PATCH 00/13] Reconcile NUMA balancing decisions with the load balancer v6

[Peter Zijlstra]

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.
> 

Mel pointed out that that patch got mutilated somewhere (my own .Sent
copy was fine), let me try again.


---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3b64ffd6c728..df588ac75bf0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2330,7 +2330,7 @@ void scheduler_ipi(void)
 	irq_exit();
 }
 
-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);
 
@@ -2372,6 +2372,17 @@ 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)
@@ -2380,11 +2391,8 @@ 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);
@@ -2568,7 +2576,15 @@ 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.
@@ -2599,15 +2615,10 @@ 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) {
@@ -2615,14 +2626,6 @@ 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);