[PATCH v2 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

[Yicong Yang <yangyicong@hisilicon.com>]

From: Barry Song <song.bao.hua@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:
                            5.17-rc1                patched
Hmean     1        324.73 (   0.00%)      378.01 *  16.41%*
Hmean     2        645.36 (   0.00%)      754.63 *  16.93%*
Hmean     4       1302.09 (   0.00%)     1507.54 *  15.78%*
Hmean     8       2612.03 (   0.00%)     2982.57 *  14.19%*
Hmean     16      5307.12 (   0.00%)     5886.66 *  10.92%*
Hmean     32      9354.22 (   0.00%)     9908.13 *   5.92%*
Hmean     64      7240.35 (   0.00%)     7278.78 *   0.53%*
Hmean     128     6186.40 (   0.00%)     6187.85 (   0.02%)

On numa 0-1:
                            5.17-rc1                patched
Hmean     1        320.01 (   0.00%)      378.44 *  18.26%*
Hmean     2        643.85 (   0.00%)      752.52 *  16.88%*
Hmean     4       1287.36 (   0.00%)     1505.62 *  16.95%*
Hmean     8       2564.60 (   0.00%)     2955.29 *  15.23%*
Hmean     16      5195.69 (   0.00%)     5814.74 *  11.91%*
Hmean     32      9769.16 (   0.00%)    10872.63 *  11.30%*
Hmean     64     15952.50 (   0.00%)    17281.98 *   8.33%*
Hmean     128    13113.77 (   0.00%)    13895.20 *   5.96%*
Hmean     256    10997.59 (   0.00%)    11244.69 *   2.25%*
Hmean     512    14623.60 (   0.00%)    15526.25 *   6.17%*

This will also help to improve the MySQL. With MySQL server
running on numa 0 and client running on numa 1, both QPS and
latency is imporved on read-write case:
                        5.17-rc1        patched
QPS-16threads        143333.2633    145077.4033(+1.22%)
QPS-24threads        195085.9367    202719.6133(+3.91%)
QPS-32threads        241165.6867      249020.74(+3.26%)
QPS-64threads        244586.8433    253387.7567(+3.60%)
avg-lat-16threads           2.23           2.19(+1.19%)
avg-lat-24threads           2.46           2.36(+3.79%)
avg-lat-36threads           2.66           2.57(+3.26%)
avg-lat-64threads           5.23           5.05(+3.44%)

Tested-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
---
 kernel/sched/fair.c | 46 +++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5146163bfabb..2f84a933aedd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6262,12 +6262,46 @@ 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, int prev_cpu, int target)
+{
+	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
+	struct sched_domain *sd = rcu_dereference(per_cpu(sd_cluster, target));
+	int cpu, idle_cpu;
+
+	/* TODO: Support SMT case while a machine with both cluster and SMT born */
+	if (!sched_smt_active() && sd) {
+		for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
+			idle_cpu = __select_idle_cpu(cpu, p);
+			if ((unsigned int)idle_cpu < nr_cpumask_bits)
+				return idle_cpu;
+		}
+
+		/* Don't ping-pong tasks in and out cluster frequently */
+		if (cpus_share_resources(target, prev_cpu))
+			return target;
+
+		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)
+{
+	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
  * average idle time for this rq (as found in rq->avg_idle).
  */
-static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool has_idle_core, int target)
+static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool has_idle_core, int prev_cpu, int target)
 {
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	int i, cpu, idle_cpu = -1, nr = INT_MAX;
@@ -6282,6 +6316,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 
 	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
 
+	idle_cpu = scan_cluster(p, prev_cpu, target);
+	if ((unsigned int)idle_cpu < nr_cpumask_bits)
+		return idle_cpu;
+
 	if (sched_feat(SIS_PROP) && !has_idle_core) {
 		u64 avg_cost, avg_idle, span_avg;
 		unsigned long now = jiffies;
@@ -6416,7 +6454,7 @@ 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;
@@ -6442,7 +6480,7 @@ 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)) {
@@ -6483,7 +6521,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 		}
 	}
 
-	i = select_idle_cpu(p, sd, has_idle_core, target);
+	i = select_idle_cpu(p, sd, has_idle_core, prev, target);
 	if ((unsigned)i < nr_cpumask_bits)
 		return i;
 
-- 
2.24.0