linux-kernel - [PATCH 2/3] sched/fair: Fixes for capacity inversion detection

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Message-Id: <20221127141742.1644023-3-qyousef@layalina.io>
Date:   Sun, 27 Nov 2022 14:17:41 +0000
From:   Qais Yousef <qyousef@...alina.io>
To:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lukasz Luba <lukasz.luba@....com>, Wei Wang <wvw@...gle.com>,
        Xuewen Yan <xuewen.yan94@...il.com>,
        Hank <han.lin@...iatek.com>,
        Jonathan JMChen <Jonathan.JMChen@...iatek.com>,
        Qais Yousef <qyousef@...alina.io>
Subject: [PATCH 2/3] sched/fair: Fixes for capacity inversion detection

Traversing the Perf Domains requires rcu_read_lock() to be held and is
conditional on sched_energy_enabled(). rcu_read_lock() is held while in
load_balance(), add an assert to ensure this is always the case.

Also skip capacity inversion detection for our own pd; which was an
error.

Fixes: 44c7b80bffc3 ("sched/fair: Detect capacity inversion")
Reported-by: Dietmar Eggemann <dietmar.eggemann@....com>
Signed-off-by: Qais Yousef (Google) <qyousef@...alina.io>
---
 kernel/sched/fair.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 89dadaafc1ec..7c0dd57e562a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8856,16 +8856,22 @@ static void update_cpu_capacity(struct sched_domain *sd, int cpu)
 	 *   * Thermal pressure will impact all cpus in this perf domain
 	 *     equally.
 	 */
-	if (static_branch_unlikely(&sched_asym_cpucapacity)) {
+	if (sched_energy_enabled()) {
 		unsigned long inv_cap = capacity_orig - thermal_load_avg(rq);
 		struct perf_domain *pd = rcu_dereference(rq->rd->pd);
 
 		rq->cpu_capacity_inverted = 0;
 
+		SCHED_WARN_ON(!rcu_read_lock_held());
+
 		for (; pd; pd = pd->next) {
 			struct cpumask *pd_span = perf_domain_span(pd);
 			unsigned long pd_cap_orig, pd_cap;
 
+			/* We can't be inverted against our own pd */
+			if (cpumask_test_cpu(cpu_of(rq), pd_span))
+				continue;
+
 			cpu = cpumask_any(pd_span);
 			pd_cap_orig = arch_scale_cpu_capacity(cpu);
 
-- 
2.25.1