sched/fair: Disable scheduler feature NEXT_BUDDY

NEXT_BUDDY was disabled with the introduction of EEVDF and enabled again
after NEXT_BUDDY was rewritten for EEVDF by commit e837456fdca8 ("sched/fair:
Reimplement NEXT_BUDDY to align with EEVDF goals"). It was not expected
that this would be a universal win without a crystal ball instruction
but the reported regressions are a concern [1][2] even if gains were
also reported. Specifically;

o mysql with client/server running on different servers regresses
o specjbb reports lower peak metrics
o daytrader regresses

The mysql is realistic and a concern. It needs to be confirmed if
specjbb is simply shifting the point where peak performance is measured
but still a concern. daytrader is considered to be representative of a
real workload.

Access to test machines is currently problematic for verifying any fix to
this problem. Disable NEXT_BUDDY for now by default until the root causes
are addressed.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Link: https://lore.kernel.org/lkml/4b96909a-f1ac-49eb-b814-97b8adda6229@arm.com
Link: https://lore.kernel.org/lkml/ec3ea66f-3a0d-4b5a-ab36-ce778f159b5b@linux.ibm.com
Link: https://patch.msgid.link/fyqsk63pkoxpeaclyqsm5nwtz3dyejplr7rg6p74xwemfzdzuu@7m7xhs5aqpqw

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 980d92bab8abfaaac610b40b71b888c08a7aa155..136a6584be79743045a2891681beb572c79de8e6 100644 (file)
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -29,7 +29,7 @@ SCHED_FEAT(PREEMPT_SHORT, true)
  * wakeup-preemption), since its likely going to consume data we
  * touched, increases cache locality.
  */
-SCHED_FEAT(NEXT_BUDDY, true)
+SCHED_FEAT(NEXT_BUDDY, false)
 
 /*
  * Allow completely ignoring cfs_rq->next; which can be set from various