linux-kernel - [PATCH v2] sched/fair: fix sgc->{min,max}

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Date:   Sat, 4 Jan 2020 21:08:28 +0800
From:   Peng Liu <iwtbavbm@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        qais.yousef@....com, morten.rasmussen@....com,
        valentin.schneider@....com
Subject: [PATCH v2] sched/fair: fix sgc->{min,max}_capacity miscalculate

commit bf475ce0a3dd ("sched/fair: Add per-CPU min capacity to
sched_group_capacity") introduced per-cpu min_capacity.

commit e3d6d0cb66f2 ("sched/fair: Add sched_group per-CPU max capacity")
introduced per-cpu max_capacity.

Here, capacity is the accumulated sum of (maybe) many CPUs' capacity.
Compare with capacity to get {min,max}_capacity makes no sense. Instead,
we should compare one by one in each iteration to get
sgc->{min,max}_capacity of the group.

Also, the only CPU in rq->sd->groups should be rq's CPU. Thus,
capacity_of(cpu_of(rq)) should be equal to rq->sd->groups->sgc->capacity.
Code can be simplified by removing the if/else.

Signed-off-by: Peng Liu <iwtbavbm@...il.com>
Reviewed-by: Valentin Schneider <valentin.schneider@....com>
---
v1: https://lkml.org/lkml/2019/12/30/502

 kernel/sched/fair.c | 26 ++++----------------------
 1 file changed, 4 insertions(+), 22 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2d170b5da0e3..e14698a8ee38 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7793,29 +7793,11 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
 		 */
 
 		for_each_cpu(cpu, sched_group_span(sdg)) {
-			struct sched_group_capacity *sgc;
-			struct rq *rq = cpu_rq(cpu);
+			unsigned long cpu_cap = capacity_of(cpu);
 
-			/*
-			 * build_sched_domains() -> init_sched_groups_capacity()
-			 * gets here before we've attached the domains to the
-			 * runqueues.
-			 *
-			 * Use capacity_of(), which is set irrespective of domains
-			 * in update_cpu_capacity().
-			 *
-			 * This avoids capacity from being 0 and
-			 * causing divide-by-zero issues on boot.
-			 */
-			if (unlikely(!rq->sd)) {
-				capacity += capacity_of(cpu);
-			} else {
-				sgc = rq->sd->groups->sgc;
-				capacity += sgc->capacity;
-			}
-
-			min_capacity = min(capacity, min_capacity);
-			max_capacity = max(capacity, max_capacity);
+			min_capacity = min(cpu_cap, min_capacity);
+			max_capacity = max(cpu_cap, max_capacity);
+			capacity += cpu_cap;
 		}
 	} else  {
 		/*
-- 
2.17.1