From: Mel Gorman <mgorman@techsingularity.net> To: LKML <linux-kernel@vger.kernel.org> Cc: Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Vincent Guittot <vincent.guittot@linaro.org>, Valentin Schneider <valentin.schneider@arm.com>, Aubrey Li <aubrey.li@linux.intel.com>, Mel Gorman <mgorman@techsingularity.net> Subject: [PATCH 4/9] sched/fair: Use prev instead of new target as recent_used_cpu Date: Mon, 26 Jul 2021 11:22:42 +0100 [thread overview] Message-ID: <20210726102247.21437-5-mgorman@techsingularity.net> (raw) In-Reply-To: <20210726102247.21437-1-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 4e2979b73cec..75ff991a460a 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6390,6 +6390,7 @@ 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) { if (cpus_share_cache(recent_used_cpu, target) && @@ -6922,9 +6923,6 @@ 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(); -- 2.26.2
next prev parent reply other threads:[~2021-07-26 10:23 UTC|newest] Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-26 10:22 [RFC PATCH 0/9] Modify and/or delete SIS_PROP Mel Gorman 2021-07-26 10:22 ` [PATCH 1/9] sched/fair: Track efficiency of select_idle_sibling Mel Gorman 2021-07-26 10:22 ` [PATCH 2/9] sched/fair: Track efficiency of task recent_used_cpu Mel Gorman 2021-07-26 10:22 ` [PATCH 3/9] sched/fair: Track efficiency of select_idle_core Mel Gorman 2021-07-26 10:22 ` Mel Gorman [this message] 2021-07-26 10:22 ` [PATCH 5/9] sched/fair: Avoid a second scan of target in select_idle_cpu Mel Gorman 2021-07-26 10:22 ` [PATCH 6/9] sched/fair: Make select_idle_cpu() proportional to cores Mel Gorman 2021-07-26 10:22 ` [PATCH 7/9] sched/fair: Enforce proportional scan limits when scanning for an idle core Mel Gorman 2021-08-02 10:52 ` Song Bao Hua (Barry Song) 2021-08-04 10:22 ` Mel Gorman 2021-07-26 10:22 ` [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup Mel Gorman 2021-08-02 10:41 ` Song Bao Hua (Barry Song) 2021-08-04 10:26 ` Mel Gorman 2021-08-05 0:23 ` Aubrey Li 2021-09-17 3:44 ` Barry Song 2021-09-17 4:15 ` Barry Song 2021-09-17 9:11 ` Aubrey Li 2021-09-17 13:35 ` Mel Gorman 2021-07-26 10:22 ` [PATCH 9/9] sched/core: Delete SIS_PROP and rely on the idle cpu mask Mel Gorman
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