[PATCH v3 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

[Yicong Yang]

From: Barry Song <song.bao.hua at hisilicon.com>

For platforms having clusters like Kunpeng920, CPUs within the same cluster
have lower latency when synchronizing and accessing shared resources like
cache. Thus, this patch tries to find an idle cpu within the cluster of the
target CPU before scanning the whole LLC to gain lower latency.

Note neither Kunpeng920 nor x86 Jacobsville supports SMT, so this patch
doesn't consider SMT for this moment.

Testing has been done on Kunpeng920 by pinning tasks to one numa and two
numa. On Kunpeng920, Each numa has 8 clusters and each cluster has 4 CPUs.

With this patch, We noticed enhancement on tbench within one numa or cross
two numa.

On numa 0:
                            5.19-rc1                patched
Hmean     1        350.27 (   0.00%)      406.88 *  16.16%*
Hmean     2        702.01 (   0.00%)      808.22 *  15.13%*
Hmean     4       1405.14 (   0.00%)     1614.34 *  14.89%*
Hmean     8       2830.53 (   0.00%)     3169.02 *  11.96%*
Hmean     16      5597.95 (   0.00%)     6224.20 *  11.19%*
Hmean     32     10537.38 (   0.00%)    10524.97 *  -0.12%*
Hmean     64      8366.04 (   0.00%)     8437.41 *   0.85%*
Hmean     128     7060.87 (   0.00%)     7150.25 *   1.27%*

On numa 0-1:
                            5.19-rc1                patched
Hmean     1        346.11 (   0.00%)      408.47 *  18.02%*
Hmean     2        693.34 (   0.00%)      805.78 *  16.22%*
Hmean     4       1384.96 (   0.00%)     1602.49 *  15.71%*
Hmean     8       2699.45 (   0.00%)     3069.98 *  13.73%*
Hmean     16      5327.11 (   0.00%)     5688.19 *   6.78%*
Hmean     32     10019.10 (   0.00%)    11862.56 *  18.40%*
Hmean     64     13850.57 (   0.00%)    17748.54 *  28.14%*
Hmean     128    12498.25 (   0.00%)    15541.59 *  24.35%*
Hmean     256    11195.77 (   0.00%)    13854.06 *  23.74%*

Tested-by: Yicong Yang <yangyicong at hisilicon.com>
Signed-off-by: Barry Song <song.bao.hua at hisilicon.com>
Signed-off-by: Yicong Yang <yangyicong at hisilicon.com>
---
 kernel/sched/fair.c | 43 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 40 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 77b2048a9326..f0496b93449c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6327,6 +6327,39 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
 
 #endif /* CONFIG_SCHED_SMT */
 
+#ifdef CONFIG_SCHED_CLUSTER
+/*
+ * Scan the cluster domain for idle CPUs and clear cluster cpumask after scanning
+ */
+static inline int scan_cluster(struct task_struct *p, struct cpumask *cpus,
+			       int target, int *nr)
+{
+	struct sched_domain *sd = rcu_dereference(per_cpu(sd_cluster, target));
+	int cpu, idle_cpu;
+
+	/* TODO: Support SMT system with cluster topology */
+	if (!sched_smt_active() && sd) {
+		for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
+			if (!--*nr)
+				break;
+
+			idle_cpu = __select_idle_cpu(cpu, p);
+			if ((unsigned int)idle_cpu < nr_cpumask_bits)
+				return idle_cpu;
+		}
+
+		cpumask_andnot(cpus, cpus, sched_domain_span(sd));
+	}
+
+	return -1;
+}
+#else
+static inline int scan_cluster(struct task_struct *p, int prev_cpu, int target, int *nr)
+{
+	return -1;
+}
+#endif
+
 /*
  * 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
@@ -6375,6 +6408,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 		time = cpu_clock(this);
 	}
 
+	idle_cpu = scan_cluster(p, cpus, target, &nr);
+	if ((unsigned int)idle_cpu < nr_cpumask_bits)
+		return idle_cpu;
+
 	for_each_cpu_wrap(cpu, cpus, target + 1) {
 		if (has_idle_core) {
 			i = select_idle_core(p, cpu, cpus, &idle_cpu);
@@ -6382,7 +6419,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 				return i;
 
 		} else {
-			if (!--nr)
+			if (--nr <= 0)
 				return -1;
 			idle_cpu = __select_idle_cpu(cpu, p);
 			if ((unsigned int)idle_cpu < nr_cpumask_bits)
@@ -6481,7 +6518,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	/*
 	 * If the previous CPU is cache affine and idle, don't be stupid:
 	 */
-	if (prev != target && cpus_share_cache(prev, target) &&
+	if (prev != target && cpus_share_resources(prev, target) &&
 	    (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
 	    asym_fits_capacity(task_util, prev))
 		return prev;
@@ -6507,7 +6544,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	p->recent_used_cpu = prev;
 	if (recent_used_cpu != prev &&
 	    recent_used_cpu != target &&
-	    cpus_share_cache(recent_used_cpu, target) &&
+	    cpus_share_resources(recent_used_cpu, target) &&
 	    (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
 	    cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) &&
 	    asym_fits_capacity(task_util, recent_used_cpu)) {
-- 
2.24.0