sched: adjust sleeper credit for SCHED_IDLE entities

Give reduced sleeper credit to SCHED_IDLE entities. As a result, woken
SCHED_IDLE entities will take longer to preempt normal entities.

The benefit of this change is to make it less likely that a newly woken
SCHED_IDLE entity will preempt a short-running normal entity before it
blocks.

We still give a small sleeper credit to SCHED_IDLE entities, so that
idle<->idle competition retains some fairness.

Example: With HZ=1000, spawned four threads affined to one cpu, one of
which was set to SCHED_IDLE. Without this patch, wakeup latency for the
SCHED_IDLE thread was ~1-2ms, with the patch the wakeup latency was
~5ms.

Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Jiang Biao <benbjiang@tencent.com>
Link: https://lore.kernel.org/r/20210820010403.946838-5-joshdon@google.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d835061a9bd2c281a33b29f5f05f3e1d14bfb4ae..5457c8001c2ac7a1e989f36b7d7ba0e9709c3fc9 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4230,7 +4230,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 
        /* sleeps up to a single latency don't count. */
        if (!initial) {
-               unsigned long thresh = sysctl_sched_latency;
+               unsigned long thresh;
+
+               if (se_is_idle(se))
+                       thresh = sysctl_sched_min_granularity;
+               else
+                       thresh = sysctl_sched_latency;
 
                /*
                 * Halve their sleep time's effect, to allow