[PATCH 3/4] sched/fair: Return an idle cpu if one is found after a failed search for an idle core

[Mel Gorman <mgorman@techsingularity.net>]

select_idle_core is called when SMT is active and there is likely a free
core available. It may find idle CPUs but this information is simply
discarded and the scan starts over again with select_idle_cpu.

This patch caches information on idle CPUs found during the search for
a core and uses one if no core is found. This is a tradeoff. There may
be a slight impact when utilisation is low and an idle core can be
found quickly. It provides improvements as the number of busy CPUs
approaches 50% of the domain size when SMT is enabled.

With tbench on a 2-socket CascadeLake machine, 80 logical CPUs, HT enabled

                          5.10.0-rc6             5.10.0-rc6
                           schedstat          idlecandidate
Hmean     1        500.06 (   0.00%)      505.67 *   1.12%*
Hmean     2        975.90 (   0.00%)      974.06 *  -0.19%*
Hmean     4       1902.95 (   0.00%)     1904.43 *   0.08%*
Hmean     8       3761.73 (   0.00%)     3721.02 *  -1.08%*
Hmean     16      6713.93 (   0.00%)     6769.17 *   0.82%*
Hmean     32     10435.31 (   0.00%)    10312.58 *  -1.18%*
Hmean     64     12325.51 (   0.00%)    13792.01 *  11.90%*
Hmean     128    21225.21 (   0.00%)    20963.44 *  -1.23%*
Hmean     256    20532.83 (   0.00%)    20335.62 *  -0.96%*
Hmean     320    20334.81 (   0.00%)    20147.25 *  -0.92%*

Note that there is a significant corner case. As the SMT scan may be
terminated early, not all CPUs have been visited and select_idle_cpu()
is still called for a full scan. This case is handled in the next
patch.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 kernel/sched/fair.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 01b38fc17bca..00c3b526a5bd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6066,6 +6066,7 @@ void __update_idle_core(struct rq *rq)
  */
 static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
 {
+	int idle_candidate = -1;
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	int core, cpu;
 
@@ -6085,6 +6086,11 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
 				idle = false;
 				break;
 			}
+
+			if (idle_candidate == -1 &&
+			    cpumask_test_cpu(cpu, p->cpus_ptr)) {
+				idle_candidate = cpu;
+			}
 		}
 
 		if (idle)
@@ -6098,7 +6104,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
 	 */
 	set_idle_cores(target, 0);
 
-	return -1;
+	return idle_candidate;
 }
 
 /*
-- 
2.26.2