linux-kernel - [PATCH v4 2/4] sched/isolation: Make "isolcpus=nohz" equivalent to "nohz

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Message-ID: <20240921190720.106195-3-longman@redhat.com>
Date: Sat, 21 Sep 2024 15:07:18 -0400
From: Waiman Long <longman@...hat.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>,
	Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org,
	Phil Auld <pauld@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: [PATCH v4 2/4] sched/isolation: Make "isolcpus=nohz" equivalent to "nohz_full"

The "isolcpus=nohz" boot parameter and flag were used to disable tick
when running a single task.  Nowsdays, this "nohz" flag is seldomly used
as it is included as part of the "nohz_full" parameter.  Extend this
flag to cover other kernel noises disabled by the "nohz_full" parameter
to make them equivalent. This also eliminates the need to use both the
"isolcpus" and the "nohz_full" parameters to fully isolated a given
set of CPUs.

Suggested-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Waiman Long <longman@...hat.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 4 +++-
 kernel/sched/isolation.c                        | 6 +++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 8337d0fed311..4e664587943f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2424,7 +2424,9 @@
 			specified in the flag list (default: domain):
 
 			nohz
-			  Disable the tick when a single task runs.
+			  Disable the tick when a single task runs as well as
+			  disabling other kernel noises like having RCU callbacks
+			  offloaded. This is equivalent to the nohz_full parameter.
 
 			  A residual 1Hz tick is offloaded to workqueues, which you
 			  need to affine to housekeeping through the global
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 5345e11f3d44..6a686322ce3c 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -209,9 +209,13 @@ static int __init housekeeping_isolcpus_setup(char *str)
 	int len;
 
 	while (isalpha(*str)) {
+		/*
+		 * isolcpus=nohz is equivalent to nohz_full.
+		 */
 		if (!strncmp(str, "nohz,", 5)) {
 			str += 5;
-			flags |= HK_FLAG_TICK;
+			flags |= HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER |
+				 HK_FLAG_RCU | HK_FLAG_MISC | HK_FLAG_KTHREAD;
 			continue;
 		}
 
-- 
2.43.5