linux-kernel - [tip:sched/core] sched/fair: Explain LLC nohz kick condition

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Date:   Mon, 11 Feb 2019 02:51:12 -0800
From:   tip-bot for Valentin Schneider <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        hpa@...or.com, peterz@...radead.org, torvalds@...ux-foundation.org,
        valentin.schneider@....com
Subject: [tip:sched/core] sched/fair: Explain LLC nohz kick condition

Commit-ID:  892d59c22208be820a5463b5f74eb7f0b7f2b03a
Gitweb:     https://git.kernel.org/tip/892d59c22208be820a5463b5f74eb7f0b7f2b03a
Author:     Valentin Schneider <valentin.schneider@....com>
AuthorDate: Thu, 17 Jan 2019 15:34:08 +0000
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 11 Feb 2019 08:02:17 +0100

sched/fair: Explain LLC nohz kick condition

Provide a comment explaining the LLC related nohz kick in
nohz_balancer_kick().

Signed-off-by: Valentin Schneider <valentin.schneider@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Dietmar.Eggemann@....com
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: morten.rasmussen@....com
Cc: vincent.guittot@...aro.org
Link: https://lkml.kernel.org/r/20190117153411.2390-3-valentin.schneider@arm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0692c8ff6ff6..ac6b52d8c79e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9601,8 +9601,13 @@ static void nohz_balancer_kick(struct rq *rq)
 	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
 	if (sds) {
 		/*
-		 * XXX: write a coherent comment on why we do this.
-		 * See also: http://lkml.kernel.org/r/20111202010832.602203411@sbsiddha-desk.sc.intel.com
+		 * If there is an imbalance between LLC domains (IOW we could
+		 * increase the overall cache use), we need some less-loaded LLC
+		 * domain to pull some load. Likewise, we may need to spread
+		 * load within the current LLC domain (e.g. packed SMT cores but
+		 * other CPUs are idle). We can't really know from here how busy
+		 * the others are - so just get a nohz balance going if it looks
+		 * like this LLC domain has tasks we could move.
 		 */
 		nr_busy = atomic_read(&sds->nr_busy_cpus);
 		if (nr_busy > 1) {