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Subject[PATCH 4/9] sched/fair: Use prev instead of new target as recent_used_cpu
After select_idle_sibling, p->recent_used_cpu is set to the
new target. However on the next wakeup, prev will be the same as
recent_used_cpu unless the load balancer has moved the task since the
last wakeup. It still works, but is less efficient than it could be.
This patch preserves recent_used_cpu for longer.

The impact on SIS efficiency is tiny so the SIS statistic patches were
used to track the hit rate for using recent_used_cpu. With perf bench
pipe on a 2-socket Cascadelake machine, the hit rate went from 57.14%
to 85.32%. For more intensive wakeup loads like hackbench, the hit rate
is almost negligible but rose from 0.21% to 6.64%. For scaling loads
like tbench, the hit rate goes from almost 0% to 25.42% overall. Broadly
speaking, on tbench, the success rate is much higher for lower thread
counts and drops to almost 0 as the workload scales to towards saturation.

Signed-off-by: Mel Gorman <>
kernel/sched/fair.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4e2979b73cec..75ff991a460a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6390,6 +6390,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)

/* Check a recently used CPU as a potential idle candidate: */
recent_used_cpu = p->recent_used_cpu;
+ p->recent_used_cpu = prev;
if (recent_used_cpu != prev && recent_used_cpu != target) {

if (cpus_share_cache(recent_used_cpu, target) &&
@@ -6922,9 +6923,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
} else if (wake_flags & WF_TTWU) { /* XXX always ? */
/* Fast path */
new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
- if (want_affine)
- current->recent_used_cpu = cpu;

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