sched: Add a few SYNC hint knobs to play with

Currently we use overlap to weaken the SYNC hint, but allow it to
set the hint as well.

 echo NO_SYNC_WAKEUP > /debug/sched_features
 echo SYNC_MORE > /debug/sched_features

preserves pipe-test behaviour without using the WF_SYNC hint.

Worth playing with on more workloads...

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

2 files changed, 21 insertions, 3 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 6766959c7f44..280892e9d85e 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1165,9 +1165,17 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
         load      = source_load(prev_cpu, idx);
         this_load = target_load(this_cpu, idx);
 
-        if (sync && (curr->se.avg_overlap > sysctl_sched_migration_cost ||
-                        p->se.avg_overlap > sysctl_sched_migration_cost))
-                sync = 0;
+        if (sync) {
+               if (sched_feat(SYNC_LESS) &&
+                   (curr->se.avg_overlap > sysctl_sched_migration_cost ||
+                    p->se.avg_overlap > sysctl_sched_migration_cost))
+                       sync = 0;
+        } else {
+                if (sched_feat(SYNC_MORE) &&
+                    (curr->se.avg_overlap < sysctl_sched_migration_cost &&
+                     p->se.avg_overlap < sysctl_sched_migration_cost))
+                        sync = 1;
+        }
 
         /*
          * If sync wakeup then subtract the (maximum possible)
diff --git a/kernel/sched_features.h b/kernel/sched_features.h
index 294e10edd3c8..70115c69c7a9 100644
--- a/kernel/sched_features.h
+++ b/kernel/sched_features.h
@@ -63,6 +63,16 @@ SCHED_FEAT(SYNC_WAKEUPS, 1)
 SCHED_FEAT(AFFINE_WAKEUPS, 1)
 
 /*
+ * Weaken SYNC hint based on overlap
+ */
+SCHED_FEAT(SYNC_LESS, 1)
+
+/*
+ * Add SYNC hint based on overlap
+ */
+SCHED_FEAT(SYNC_MORE, 0)
+
+/*
  * Prefer to schedule the task we woke last (assuming it failed
  * wakeup-preemption), since its likely going to consume data we
  * touched, increases cache locality.