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From:   "Joel Fernandes (Google)" <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Suleiman Souhlal <suleiman@google.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH 2/3] sched/nohz: Update comments about NEWILB_KICK
Date:   Fri, 20 Oct 2023 01:40:27 +0000
Message-ID: <20231020014031.919742-2-joel@joelfernandes.org>
In-Reply-To: <20231020014031.919742-1-joel@joelfernandes.org>
References: <20231020014031.919742-1-joel@joelfernandes.org>
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How ILB is triggered without IPIs is cryptic. Out of mercy for future
code readers, document it in code comments.

The comments are derived from a discussion with Vincent in a past
review.

Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/fair.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 965c30fbbe5c..8e276d12c3cb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11959,8 +11959,19 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 }
 
 /*
- * Check if we need to run the ILB for updating blocked load before entering
- * idle state.
+ * Check if we need to directly run the ILB for updating blocked load before
+ * entering idle state. Here we run ILB directly without issuing IPIs.
+ *
+ * Note that when this function is called, the tick may not yet be stopped on
+ * this CPU yet. nohz.idle_cpus_mask is updated only when tick is stopped and
+ * cleared on the next busy tick. In other words, nohz.idle_cpus_mask updates
+ * don't align with CPUs enter/exit idle to avoid bottlenecks due to high idle
+ * entry/exit rate (usec). So it is possible that _nohz_idle_balance() is
+ * called from this function on (this) CPU that's not yet in the mask. That's
+ * OK because the goal of nohz_run_idle_balance() is to run ILB only for
+ * updating the blocked load of already idle CPUs without waking up one of
+ * those idle CPUs and outside the preempt disable / irq off phase of the local
+ * cpu about to enter idle, because it can take a long time.
  */
 void nohz_run_idle_balance(int cpu)
 {
-- 
2.42.0.655.g421f12c284-goog