[tip: sched/core] sched: Disable TTWU_QUEUE on RT

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     e3865866752e1b9fd26383f548dea58334fe6eba
Gitweb:        https://git.kernel.org/tip/e3865866752e1b9fd26383f548dea58334fe6eba
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 28 Sep 2021 14:24:27 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 01 Oct 2021 13:58:07 +02:00

sched: Disable TTWU_QUEUE on RT

The queued remote wakeup mechanism has turned out to be suboptimal for RT
enabled kernels. The maximum latencies go up by a factor of > 5x in certain
scenarious.

This is caused by either long wake lists or by a large number of TTWU IPIs
which are processed back to back.

Disable it for RT.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210928122411.482262764@xxxxxxxxxxxxx
---
 kernel/sched/features.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 7f8dace..1cf435b 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -46,11 +46,16 @@ SCHED_FEAT(DOUBLE_TICK, false)
  */
 SCHED_FEAT(NONTASK_CAPACITY, true)
 
+#ifdef CONFIG_PREEMPT_RT
+SCHED_FEAT(TTWU_QUEUE, false)
+#else
+
 /*
  * Queue remote wakeups on the target CPU and process them
  * using the scheduler IPI. Reduces rq->lock contention/bounces.
  */
 SCHED_FEAT(TTWU_QUEUE, true)
+#endif
 
 /*
  * When doing wakeups, attempt to limit superfluous scans of the LLC domain.