From e2bbf11b9658818b7303380a7d200d729cd8804d Mon Sep 17 00:00:00 2001
From: Tianchen Ding <dtcccc@linux.alibaba.com>
Date: Fri, 4 Nov 2022 10:36:01 +0800
Subject: [PATCH] sched: Clear ttwu_pending after enqueue_task()
References: git fixes (sched/core)
Patch-mainline: v6.2-rc1
Git-commit: d6962c4fe8f96f7d384d6489b6b5ab5bf3e35991
We found a long tail latency in schbench whem m*t is close to nr_cpus.
(e.g., "schbench -m 2 -t 16" on a machine with 32 cpus.)
This is because when the wakee cpu is idle, rq->ttwu_pending is cleared
too early, and idle_cpu() will return true until the wakee task enqueued.
This will mislead the waker when selecting idle cpu, and wake multiple
worker threads on the same wakee cpu. This situation is enlarged by
commit f3dd3f674555 ("sched: Remove the limitation of WF_ON_CPU on
wakelist if wakee cpu is idle") because it tends to use wakelist.
Here is the result of "schbench -m 2 -t 16" on a VM with 32vcpu
(Intel(R) Xeon(R) Platinum 8369B).
Latency percentiles (usec):
base base+revert_f3dd3f674555 base+this_patch
50.0000th: 9 13 9
75.0000th: 12 19 12
90.0000th: 15 22 15
95.0000th: 18 24 17
*99.0000th: 27 31 24
99.5000th: 3364 33 27
99.9000th: 12560 36 30
We also tested on unixbench and hackbench, and saw no performance
change.
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20221104023601.12844-1-dtcccc@linux.alibaba.com
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
kernel/sched/core.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f8ca1493268d..2db7ed8442d1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3485,13 +3485,6 @@ void sched_ttwu_pending(void *arg)
if (!llist)
return;
- /*
- * rq::ttwu_pending racy indication of out-standing wakeups.
- * Races such that false-negatives are possible, since they
- * are shorter lived that false-positives would be.
- */
- WRITE_ONCE(rq->ttwu_pending, 0);
-
rq_lock_irqsave(rq, &rf);
update_rq_clock(rq);
@@ -3505,6 +3498,17 @@ void sched_ttwu_pending(void *arg)
ttwu_do_activate(rq, p, p->sched_remote_wakeup ? WF_MIGRATED : 0, &rf);
}
+ /*
+ * Must be after enqueueing at least once task such that
+ * idle_cpu() does not observe a false-negative -- if it does,
+ * it is possible for select_idle_siblings() to stack a number
+ * of tasks on this CPU during that window.
+ *
+ * It is ok to clear ttwu_pending when another task pending.
+ * We will receive IPI after local irq enabled and then enqueue it.
+ * Since now nr_running > 0, idle_cpu() will always get correct result.
+ */
+ WRITE_ONCE(rq->ttwu_pending, 0);
rq_unlock_irqrestore(rq, &rf);
}