RE: [RFC PATCH v4 2/3] scheduler: add scheduler level for clusters

quoted

-----Original Message-----
From: Peter Zijlstra [mailto:peterz@infradead.org]
Sent: Tuesday, March 2, 2021 11:43 PM
To: Song Bao Hua (Barry Song) <redacted>
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Subject: Re: [RFC PATCH v4 2/3] scheduler: add scheduler level for clusters

On Tue, Mar 02, 2021 at 11:59:39AM +1300, Barry Song wrote:

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 88a2e2b..d805e59 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7797,6 +7797,16 @@ int sched_cpu_activate(unsigned int cpu)
 	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
 		static_branch_inc_cpuslocked(&sched_smt_present);
 #endif
+
+#ifdef CONFIG_SCHED_CLUSTER
+	/*
+	 * When going up, increment the number of cluster cpus with
+	 * cluster present.
+	 */
+	if (cpumask_weight(cpu_cluster_mask(cpu)) > 1)
+		static_branch_inc_cpuslocked(&sched_cluster_present);
+#endif
+
 	set_cpu_active(cpu, true);

 	if (sched_smp_initialized) {
@@ -7873,6 +7883,14 @@ int sched_cpu_deactivate(unsigned int cpu)
 		static_branch_dec_cpuslocked(&sched_smt_present);
 #endif

+#ifdef CONFIG_SCHED_CLUSTER
+	/*
+	 * When going down, decrement the number of cpus with cluster present.
+	 */
+	if (cpumask_weight(cpu_cluster_mask(cpu)) > 1)
+		static_branch_dec_cpuslocked(&sched_cluster_present);
+#endif
+
 	if (!sched_smp_initialized)
 		return 0;
I don't think that's correct. IIUC this will mean the
sched_cluster_present thing will be enabled on anything with SMT (very
much including x86 big cores after the next patch).

I'm thinking that at the very least you should check a CLS domain
exists, but that might be hard at this point, because the sched domains
haven't been build yet.
might be able to achieve the same goal by:

int cls_wt = cpumask_weight(cpu_cluster_mask(cpu));
if ((cls_wt > cpumask_weight(cpu_smt_mask(cpu))) &&
     && (cls_wt < cpumask_weight(cpu_coregroup_mask(cpu))))
   sched_cluster_present...


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8a8bd7b..3db7b07 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6009,6 +6009,11 @@ static inline int __select_idle_cpu(int cpu)
 	return -1;
 }

+#ifdef CONFIG_SCHED_CLUSTER
+DEFINE_STATIC_KEY_FALSE(sched_cluster_present);
+EXPORT_SYMBOL_GPL(sched_cluster_present);
I really rather think this shouldn't be exported
Ok. Make sense.


+#endif
+
 #ifdef CONFIG_SCHED_SMT
 DEFINE_STATIC_KEY_FALSE(sched_smt_present);
 EXPORT_SYMBOL_GPL(sched_smt_present);
This is a KVM wart, it needs to know because mitigation crap.
Ok.

@@ -6116,6 +6121,26 @@ static inline int select_idle_core(struct task_struct
*p, int core, struct cpuma
 #endif /* CONFIG_SCHED_SMT */

+static inline int _select_idle_cpu(bool smt, struct task_struct *p, int
target, struct cpumask *cpus, int *idle_cpu, int *nr)
+{
+	int cpu, i;
+
+	for_each_cpu_wrap(cpu, cpus, target) {
+		if (smt) {
+			i = select_idle_core(p, cpu, cpus, idle_cpu);
+		} else {
+			if (!--*nr)
+				return -1;
+			i = __select_idle_cpu(cpu);
+		}
+
+		if ((unsigned int)i < nr_cpumask_bits)
+			return i;
+	}
+
+	return -1;
+}
+
 /*
  * Scan the LLC domain for idle CPUs; this is dynamically regulated by
  * comparing the average scan cost (tracked in sd->avg_scan_cost) against
the
@@ -6124,7 +6149,7 @@ static inline int select_idle_core(struct task_struct
*p, int core, struct cpuma
 static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd,
int target)
 {
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
-	int i, cpu, idle_cpu = -1, nr = INT_MAX;
+	int i, idle_cpu = -1, nr = INT_MAX;
 	bool smt = test_idle_cores(target, false);
 	int this = smp_processor_id();
 	struct sched_domain *this_sd;
@@ -6134,7 +6159,12 @@ static int select_idle_cpu(struct task_struct *p,
struct sched_domain *sd, int t
 	if (!this_sd)
 		return -1;

-	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+	if (!sched_cluster_active())
+		cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+#ifdef CONFIG_SCHED_CLUSTER
+	if (sched_cluster_active())
+		cpumask_and(cpus, cpu_cluster_mask(target), p->cpus_ptr);
+#endif

 	if (sched_feat(SIS_PROP) && !smt) {
 		u64 avg_cost, avg_idle, span_avg;
@@ -6155,24 +6185,32 @@ static int select_idle_cpu(struct task_struct *p,
struct sched_domain *sd, int t
 		time = cpu_clock(this);
 	}

-	for_each_cpu_wrap(cpu, cpus, target) {
-		if (smt) {
-			i = select_idle_core(p, cpu, cpus, &idle_cpu);
-			if ((unsigned int)i < nr_cpumask_bits)
-				return i;
+	/* scan cluster before scanning the whole llc */
+#ifdef CONFIG_SCHED_CLUSTER
+	if (sched_cluster_active()) {
+		i = _select_idle_cpu(smt, p, target, cpus, &idle_cpu, &nr);
+		if ((unsigned int) i < nr_cpumask_bits) {
+			idle_cpu = i;
+			goto done;
+		} else if (nr <= 0)
+			return -1;

-		} else {
-			if (!--nr)
-				return -1;
-			idle_cpu = __select_idle_cpu(cpu);
-			if ((unsigned int)idle_cpu < nr_cpumask_bits)
-				break;
-		}
+		cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+		cpumask_andnot(cpus, cpus, cpu_cluster_mask(target));
 	}
+#endif
+
+	i = _select_idle_cpu(smt, p, target, cpus, &idle_cpu, &nr);
+	if ((unsigned int) i < nr_cpumask_bits) {
+		idle_cpu = i;
+		goto done;
+	} else if (nr <= 0)
+		return -1;

 	if (smt)
 		set_idle_cores(this, false);

+done:
 	if (sched_feat(SIS_PROP) && !smt) {
 		time = cpu_clock(this) - time;
 		update_avg(&this_sd->avg_scan_cost, time);
And this is just horrific :-(
I was actually quite struggling with this part.
Had tried a couple of ways before sending this.
Still the sent one was quite ugly.

Thanks
Barry


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