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 [thread overview]
Message-ID: <20231020014031.919742-2-joel@joelfernandes.org> (raw)
In-Reply-To: <20231020014031.919742-1-joel@joelfernandes.org>
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
next prev parent reply other threads:[~2023-10-20 1:40 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-20 1:40 [PATCH 1/3] sched/nohz: Update nohz.next_balance directly without IPIs (v2) Joel Fernandes (Google)
2023-10-20 1:40 ` Joel Fernandes (Google) [this message]
2023-10-20 7:51 ` [PATCH 2/3] sched/nohz: Update comments about NEWILB_KICK Ingo Molnar
2023-10-20 8:02 ` [tip: sched/core] " tip-bot2 for Joel Fernandes (Google)
2023-10-20 1:40 ` [PATCH 3/3] sched: Update ->next_balance correctly during newidle balance Joel Fernandes (Google)
2023-10-20 7:53 ` Ingo Molnar
2023-10-20 13:48 ` Vincent Guittot
2023-10-21 6:50 ` Ingo Molnar
2023-10-20 8:02 ` [tip: sched/core] sched/fair: " tip-bot2 for Vineeth Pillai (Google)
2023-10-20 13:40 ` [PATCH 3/3] sched: " Vincent Guittot
2023-10-20 13:56 ` Ingo Molnar
2023-10-20 15:50 ` Joel Fernandes
2023-10-22 0:28 ` Joel Fernandes
2023-10-26 14:23 ` Vincent Guittot
2023-11-09 10:02 ` Joel Fernandes
2023-11-09 12:31 ` Joel Fernandes
2023-11-14 15:43 ` Vincent Guittot
2023-11-14 17:43 ` Joel Fernandes
2023-10-20 4:17 ` [PATCH 1/3] sched/nohz: Update nohz.next_balance directly without IPIs (v2) Joel Fernandes
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