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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 13c3fd40bd34b9ed1531981fd364fb9d452eaa2b..7ee394b4b4cdc9f26bc48a10fe00b347d9a72606 100644 (file)
--- 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();