lkml.org 
[lkml]   [2022]   [Nov]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] sched: Clear ttwu_pending after enqueue_task
Date
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

Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
---
kernel/sched/core.c | 8 +-------
1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 87c9cdf37a26..b07de1753be5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3739,13 +3739,6 @@ 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);

@@ -3760,6 +3753,7 @@ void sched_ttwu_pending(void *arg)
}

rq_unlock_irqrestore(rq, &rf);
+ WRITE_ONCE(rq->ttwu_pending, 0);
}

void send_call_function_single_ipi(int cpu)
--
2.27.0
\
 
 \ /
  Last update: 2022-11-01 08:37    [W:0.134 / U:0.284 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site