[tip: sched/core] sched/fair: Use prev instead of new target as recent_used_cpu

[tip-bot2 for Mel Gorman]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     89aafd67f28c9e3b725aa30b44b7f61ad3e348ce
Gitweb:        https://git.kernel.org/tip/89aafd67f28c9e3b725aa30b44b7f61ad3e348ce
Author:        Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
AuthorDate:    Wed, 04 Aug 2021 12:58:56 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 04 Aug 2021 15:16:44 +02:00

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 <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210804115857.6253-2-mgorman@xxxxxxxxxxxxxxxxxxx
---
 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 13c3fd4..7ee394b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6347,6 +6347,7 @@ 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 &&
 	    cpus_share_cache(recent_used_cpu, target) &&
@@ -6873,9 +6874,6 @@ 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;
 	}
 	rcu_read_unlock();