sched: Implement a gentler fair-sleepers feature

Add back FAIR_SLEEPERS and GENTLE_FAIR_SLEEPERS.

FAIR_SLEEPERS is the old logic: credit sleepers with their sleep time.

GENTLE_FAIR_SLEEPERS dampens this a bit: 50% of their sleep time gets
credited.

The hope here is to still give the benefits of fair-sleepers logic
(quick wakeups, etc.) while not allow them to have 100% of their
sleep time as if they were running.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 16 insertions, 2 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index a37f311f436e..acf16a8d934b 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -711,7 +711,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 
         if (!initial) {
                 /* sleeps upto a single latency don't count. */
-                if (sched_feat(NEW_FAIR_SLEEPERS)) {
+                if (sched_feat(FAIR_SLEEPERS)) {
                         unsigned long thresh = sysctl_sched_latency;
 
                         /*
@@ -725,6 +725,13 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
                                          task_of(se)->policy != SCHED_IDLE))
                                 thresh = calc_delta_fair(thresh, se);
 
+                        /*
+                         * Halve their sleep time's effect, to allow
+                         * for a gentler effect of sleepers:
+                         */
+                        if (sched_feat(GENTLE_FAIR_SLEEPERS))
+                                thresh >>= 1;
+
                         vruntime -= thresh;
                 }
         }
diff --git a/kernel/sched_features.h b/kernel/sched_features.h
index 70115c69c7a9..fd375675f834 100644
--- a/kernel/sched_features.h
+++ b/kernel/sched_features.h
@@ -3,7 +3,14 @@
  * considers the task to be running during that period. This gives it
  * a service deficit on wakeup, allowing it to run sooner.
  */
-SCHED_FEAT(NEW_FAIR_SLEEPERS, 0)
+SCHED_FEAT(FAIR_SLEEPERS, 1)
+
+/*
+ * Only give sleepers 50% of their service deficit. This allows
+ * them to run sooner, but does not allow tons of sleepers to
+ * rip the spread apart.
+ */
+SCHED_FEAT(GENTLE_FAIR_SLEEPERS, 1)
 
 /*
  * By not normalizing the sleep time, heavy tasks get an effective