linux-kernel - [PATCH] sched/rt: use for_each_cpu_wrap to iterate over rto

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Message-ID: <20241114220558.1842569-1-jon@nutanix.com>
Date: Thu, 14 Nov 2024 15:05:58 -0700
From: Jon Kohler <jon@...anix.com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org
Cc: Jon Kohler <jon@...anix.com>
Subject: [PATCH] sched/rt: use for_each_cpu_wrap to iterate over rto_mask

When using NO_RT_PUSH_IPI, using for_each_cpu() over rto_mask may cause
many CPUs to attempt to pull load from the same CPU, causing RQ
lock contention.

Use for_each_cpu_wrap instead to spread out which RQ gets evaluated
first, similar to how _nohz_idle_balance iterates over idle_cpus_mask.
This strategy is beneficial when there are many CPUs in rto_mask and
many other CPUs going in and out of schedule() at the same time.

Signed-off-by: Jon Kohler <jon@...anix.com>
---
 kernel/sched/rt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 172c588de542..c883ff122f5d 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2308,7 +2308,7 @@ static void pull_rt_task(struct rq *this_rq)
 	}
 #endif
 
-	for_each_cpu(cpu, this_rq->rd->rto_mask) {
+	for_each_cpu_wrap(cpu, this_rq->rd->rto_mask, this_cpu+1) {
 		if (this_cpu == cpu)
 			continue;
 
-- 
2.43.0