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Date:   Thu, 12 Dec 2019 16:24:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Cheng Jian <cj.chengjian@huawei.com>
Cc:     mingo@kernel.org, linux-kernel@vger.kernel.org,
        chenwandun@huawei.com, xiexiuqi@huawei.com, liwei391@huawei.com,
        huawei.libin@huawei.com, bobo.shaobowang@huawei.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org
Subject: Re: [PATCH] sched/fair: Optimize select_idle_cpu
Message-ID: <20191212152406.GB2827@hirez.programming.kicks-ass.net>
References: <20191212144102.181510-1-cj.chengjian@huawei.com>
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On Thu, Dec 12, 2019 at 10:41:02PM +0800, Cheng Jian wrote:

> Fixes: 1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")

The 'funny' thing is that select_idle_core() actually does the right
thing.

Copying that should work:


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 08a233e97a01..416d574dcebf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5828,6 +5837,7 @@ static inline int select_idle_smt(struct task_struct *p, int target)
  */
 static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
 {
+	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	struct sched_domain *this_sd;
 	u64 avg_cost, avg_idle;
 	u64 time, cost;
@@ -5859,11 +5869,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 
 	time = cpu_clock(this);
 
-	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
+	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+
+	for_each_cpu_wrap(cpu, cpus, target) {
 		if (!--nr)
 			return si_cpu;
-		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
-			continue;
 		if (available_idle_cpu(cpu))
 			break;
 		if (si_cpu == -1 && sched_idle_cpu(cpu))