linux-kernel - [PATCH 1/3] sched/nohz: Update idle load-balancing (ILB) comments

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Message-Id: <20231006102518.2452758-2-mingo@kernel.org>
Date:   Fri,  6 Oct 2023 12:25:16 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 1/3] sched/nohz: Update idle load-balancing (ILB) comments

 - Fix incorrect/misleading comments,

 - clarify some others,

 - fix typos & grammar,

 - and use more consistent style throughout.

Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org
---
 kernel/sched/fair.c | 30 ++++++++++++++++--------------
 1 file changed, 16 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04fbcbda97d5..8435179779e3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11547,14 +11547,15 @@ static inline int on_null_domain(struct rq *rq)
 
 #ifdef CONFIG_NO_HZ_COMMON
 /*
- * idle load balancing details
- * - When one of the busy CPUs notice that there may be an idle rebalancing
+ * NOHZ idle load balancing (ILB) details:
+ *
+ * - When one of the busy CPUs notices that there may be an idle rebalancing
  *   needed, they will kick the idle load balancer, which then does idle
  *   load balancing for all the idle CPUs.
- * - HK_TYPE_MISC CPUs are used for this task, because HK_TYPE_SCHED not set
+ *
+ * - HK_TYPE_MISC CPUs are used for this task, because HK_TYPE_SCHED is not set
  *   anywhere yet.
  */
-
 static inline int find_new_ilb(void)
 {
 	int ilb;
@@ -11575,8 +11576,10 @@ static inline int find_new_ilb(void)
 }
 
 /*
- * Kick a CPU to do the nohz balancing, if it is time for it. We pick any
- * idle CPU in the HK_TYPE_MISC housekeeping set (if there is one).
+ * Kick a CPU to do the NOHZ balancing, if it is time for it, via a cross-CPU
+ * SMP function call (IPI).
+ *
+ * We pick the first idle CPU in the HK_TYPE_MISC housekeeping set (if there is one).
  */
 static void kick_ilb(unsigned int flags)
 {
@@ -11604,7 +11607,7 @@ static void kick_ilb(unsigned int flags)
 
 	/*
 	 * This way we generate an IPI on the target CPU which
-	 * is idle. And the softirq performing nohz idle load balance
+	 * is idle, and the softirq performing NOHZ idle load balancing
 	 * will be run before returning from the IPI.
 	 */
 	smp_call_function_single_async(ilb_cpu, &cpu_rq(ilb_cpu)->nohz_csd);
@@ -11633,7 +11636,7 @@ static void nohz_balancer_kick(struct rq *rq)
 
 	/*
 	 * None are in tickless mode and hence no need for NOHZ idle load
-	 * balancing.
+	 * balancing:
 	 */
 	if (likely(!atomic_read(&nohz.nr_cpus)))
 		return;
@@ -11655,9 +11658,8 @@ static void nohz_balancer_kick(struct rq *rq)
 	sd = rcu_dereference(rq->sd);
 	if (sd) {
 		/*
-		 * If there's a CFS task and the current CPU has reduced
-		 * capacity; kick the ILB to see if there's a better CPU to run
-		 * on.
+		 * If there's a runnable CFS task and the current CPU has reduced
+		 * capacity, kick the ILB to see if there's a better CPU to run on:
 		 */
 		if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
 			flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
@@ -11709,11 +11711,11 @@ static void nohz_balancer_kick(struct rq *rq)
 	if (sds) {
 		/*
 		 * 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
+		 * increase the overall cache utilization), we need a less-loaded LLC
+		 * domain to pull some load from. 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
+		 * 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);
-- 
2.39.2