[tip: sched/core] sched/isolation: Make "isolcpus=nohz" equivalent to "nohz_full"

[tip-bot2 for Waiman Long]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     1174b9344bc7e7989439cad207fcd94eaab028db
Gitweb:        https://git.kernel.org/tip/1174b9344bc7e7989439cad207fcd94eaab028db
Author:        Waiman Long <longman@xxxxxxxxxx>
AuthorDate:    Wed, 30 Oct 2024 13:52:51 -04:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 02 Dec 2024 12:24:28 +01:00

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@xxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20241030175253.125248-3-longman@xxxxxxxxxx
---
 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 3872bc6..3fa0b4e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2432,7 +2432,9 @@
 			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 5345e11..6a68632 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;
 		}