From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 28 Sep 2021 12:24:27 +0000 (+0200)
Subject: sched: Disable TTWU_QUEUE on RT
X-Git-Tag: io_uring-5.16-2021-11-09~42^2~37
X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=539fbb5be0da56ffa1434b4f56521a0522bd1d61;p=linux-block.git

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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210928122411.482262764@linutronix.de
---

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 7f8dace0964c..1cf435bbcd9c 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.