linux-kernel - [RFC][PATCH 1/5] sched/fair: Fix select_idle

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Date:   Mon, 14 Dec 2020 17:48:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mgorman@...hsingularity.net, vincent.guittot@...aro.org
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        aubrey.li@...ux.intel.com, mingo@...hat.com, juri.lelli@...hat.com,
        valentin.schneider@....com, qais.yousef@....com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        tim.c.chen@...ux.intel.com, benbjiang@...il.com
Subject: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost accounting

We compute the average cost of the total scan, but then use it as a
per-cpu scan cost when computing the scan proportion. Fix this by
properly computing a per-cpu scan cost.

This also fixes a bug where we would terminate early (!--nr, case) and
not account that cost at all.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/fair.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6144,10 +6144,10 @@ static inline int select_idle_smt(struct
 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);
+	int cpu, loops = 1, nr = INT_MAX;
+	int this = smp_processor_id();
 	struct sched_domain *this_sd;
 	u64 time;
-	int this = smp_processor_id();
-	int cpu, nr = INT_MAX;
 
 	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
 	if (!this_sd)
@@ -6175,14 +6175,19 @@ static int select_idle_cpu(struct task_s
 	}
 
 	for_each_cpu_wrap(cpu, cpus, target) {
-		if (!--nr)
-			return -1;
 		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
 			break;
+
+		if (loops >= nr) {
+			cpu = -1;
+			break;
+		}
+		loops++;
 	}
 
 	if (sched_feat(SIS_PROP)) {
 		time = cpu_clock(this) - time;
+		time = div_u64(time, loops);
 		update_avg(&this_sd->avg_scan_cost, time);
 	}