[PATCH 10/10] sched/fair: Avoid revisiting CPUs multiple times during select_idle_sibling

[Mel Gorman]

Note: While this is done in the context of select_idle_core(), I would not
	expect it to be done like this. The intent is to illustrate how
	idle_cpu_mask could be filtered before select_idle_cpus() scans
	the rest of a domain or a wider scan was done across a cluster.

select_idle_core() potentially searches a number of CPUs for idle candidates
before select_idle_cpu() clears the mask and revisits the same CPUs. This
patch moves the initialisation of select_idle_mask to the top-level and
reuses the same mask across both select_idle_core and select_idle_cpu.
select_idle_smt() is left alone as the cost of checking one SMT sibling
is marginal relative to calling __clear_cpumask_cpu() for evey CPU
visited by select_idle_core().

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cd95daf9f53e..af2e108c20c0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6096,10 +6096,9 @@ void __update_idle_core(struct rq *rq)
  * sd_llc->shared->has_idle_cores and enabled through update_idle_core() above.
  */
 static int select_idle_core(struct task_struct *p, struct sched_domain *sd,
-							int target, int nr)
+				int target, int nr, struct cpumask *cpus)
 {
 	int idle_candidate = -1;
-	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	int core, cpu;
 
 	if (!static_branch_likely(&sched_smt_present))
@@ -6108,9 +6107,6 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd,
 	if (!test_idle_cores(target, false))
 		return -1;
 
-	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
-	__cpumask_clear_cpu(target, cpus);
-
 	for_each_cpu_wrap(core, cpus, target) {
 		bool idle = true;
 
@@ -6175,7 +6171,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
 #else /* CONFIG_SCHED_SMT */
 
 static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd,
-							int target, int nr)
+					int target, int nr, struct cpumask *cpus)
 {
 	return -1;
 }
@@ -6193,14 +6189,10 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
  * 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,
-							int target, int nr)
+				int target, int nr, struct cpumask *cpus)
 {
-	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	int cpu;
 
-	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
-	__cpumask_clear_cpu(target, cpus);
-
 	for_each_cpu_wrap(cpu, cpus, target) {
 		schedstat_inc(this_rq()->sis_scanned);
 		if (!--nr)
@@ -6260,6 +6252,7 @@ static inline bool asym_fits_capacity(int task_util, int cpu)
 static int select_idle_sibling(struct task_struct *p, int prev, int target)
 {
 	struct sched_domain *sd, *this_sd;
+	struct cpumask *cpus_visited;
 	unsigned long task_util;
 	int i, recent_used_cpu, depth;
 	u64 time;
@@ -6358,13 +6351,23 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 
 	depth = sis_search_depth(sd, this_sd);
 
+	/*
+	 * Init the select_idle_mask. select_idle_core() will mask
+	 * out the CPUs that have already been limited to limit the
+	 * search in select_idle_cpu(). Further clearing is not
+	 * done as select_idle_smt checks only one CPU.
+	 */
+	cpus_visited = this_cpu_cpumask_var_ptr(select_idle_mask);
+	cpumask_and(cpus_visited, sched_domain_span(sd), p->cpus_ptr);
+	__cpumask_clear_cpu(target, cpus_visited);
+
 	schedstat_inc(this_rq()->sis_domain_search);
-	i = select_idle_core(p, sd, target, depth);
+	i = select_idle_core(p, sd, target, depth, cpus_visited);
 	if ((unsigned)i < nr_cpumask_bits)
 		return i;
 
 	time = cpu_clock(smp_processor_id());
-	i = select_idle_cpu(p, sd, target, depth);
+	i = select_idle_cpu(p, sd, target, depth, cpus_visited);
 	if ((unsigned)i < nr_cpumask_bits)
 		goto acct_cost;
 
-- 
2.26.2