[PATCH v3 4/4] sched: adjust sleeper credit for SCHED_IDLE entities

[Josh Don <joshdon@google.com>]

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>
---
 kernel/sched/fair.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 31f40aa005b9..aa9c046d2aab 100644
--- 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
-- 
2.33.0.rc2.250.ged5fa647cd-goog