From: Mel Gorman <mgorman@techsingularity.net>
To: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Valentin Schneider <valentin.schneider@arm.com>,
Song Bao Hua <song.bao.hua@hisilicon.com>,
LKML <linux-kernel@vger.kernel.org>,
Mel Gorman <mgorman@techsingularity.net>
Subject: [PATCH 1/2] sched/fair: Use prev instead of new target as recent_used_cpu
Date: Wed, 4 Aug 2021 12:58:56 +0100 [thread overview]
Message-ID: <20210804115857.6253-2-mgorman@techsingularity.net> (raw)
In-Reply-To: <20210804115857.6253-1-mgorman@techsingularity.net>
After select_idle_sibling, p->recent_used_cpu is set to the
new target. However on the next wakeup, prev will be the same as
recent_used_cpu unless the load balancer has moved the task since the
last wakeup. It still works, but is less efficient than it could be.
This patch preserves recent_used_cpu for longer.
The impact on SIS efficiency is tiny so the SIS statistic patches were
used to track the hit rate for using recent_used_cpu. With perf bench
pipe on a 2-socket Cascadelake machine, the hit rate went from 57.14%
to 85.32%. For more intensive wakeup loads like hackbench, the hit rate
is almost negligible but rose from 0.21% to 6.64%. For scaling loads
like tbench, the hit rate goes from almost 0% to 25.42% overall. Broadly
speaking, on tbench, the success rate is much higher for lower thread
counts and drops to almost 0 as the workload scales to towards saturation.
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
kernel/sched/fair.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 44c452072a1b..8ad7666f387c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6376,6 +6376,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
/* Check a recently used CPU as a potential idle candidate: */
recent_used_cpu = p->recent_used_cpu;
+ p->recent_used_cpu = prev;
if (recent_used_cpu != prev &&
recent_used_cpu != target &&
cpus_share_cache(recent_used_cpu, target) &&
@@ -6902,9 +6903,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
} else if (wake_flags & WF_TTWU) { /* XXX always ? */
/* Fast path */
new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
-
- if (want_affine)
- current->recent_used_cpu = cpu;
}
rcu_read_unlock();
--
2.31.1
next prev parent reply other threads:[~2021-08-04 11:59 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-04 11:58 [PATCH 0/2] Reduce SIS scanning Mel Gorman
2021-08-04 11:58 ` Mel Gorman [this message]
2021-08-05 9:34 ` [tip: sched/core] sched/fair: Use prev instead of new target as recent_used_cpu tip-bot2 for Mel Gorman
2021-08-04 11:58 ` [PATCH 2/2] sched/fair: Avoid a second scan of target in select_idle_cpu Mel Gorman
2021-08-05 9:34 ` [tip: sched/core] " tip-bot2 for Mel Gorman
2021-08-04 13:11 ` [PATCH 0/2] Reduce SIS scanning Peter Zijlstra
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