linux-kernel - [PATCH v2 3/7] sched: Check cpu_active() earlier in newidle

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Date:   Mon, 10 Jul 2023 15:03:38 -0500
From:   David Vernet <void@...ifault.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,
        bristot@...hat.com, vschneid@...hat.com, gautham.shenoy@....com,
        kprateek.nayak@....com, aaron.lu@...el.com, clm@...a.com,
        tj@...nel.org, roman.gushchin@...ux.dev, kernel-team@...a.com
Subject: [PATCH v2 3/7] sched: Check cpu_active() earlier in newidle_balance()

In newidle_balance(), we check if the current CPU is inactive, and then
decline to pull any remote tasks to the core if so. Before this check,
however, we're currently updating rq->idle_stamp. If a core is offline,
setting its idle stamp is not useful. The core won't be chosen by any
task in select_task_rq_fair(), and setting the rq->idle_stamp is
misleading anyways given that the core being inactive should imply that
it should have a very cold cache.

Let's set rq->idle_stamp in newidle_balance() only if the cpu is active.

Signed-off-by: David Vernet <void@...ifault.com>
---
 kernel/sched/fair.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a80a73909dc2..6e882b7bf5b4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11837,18 +11837,18 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
 	if (this_rq->ttwu_pending)
 		return 0;
 
-	/*
-	 * We must set idle_stamp _before_ calling idle_balance(), such that we
-	 * measure the duration of idle_balance() as idle time.
-	 */
-	this_rq->idle_stamp = rq_clock(this_rq);
-
 	/*
 	 * Do not pull tasks towards !active CPUs...
 	 */
 	if (!cpu_active(this_cpu))
 		return 0;
 
+	/*
+	 * We must set idle_stamp _before_ calling idle_balance(), such that we
+	 * measure the duration of idle_balance() as idle time.
+	 */
+	this_rq->idle_stamp = rq_clock(this_rq);
+
 	/*
 	 * This is OK, because current is on_cpu, which avoids it being picked
 	 * for load-balance and preemption/IRQs are still disabled avoiding
-- 
2.40.1