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 <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Frederic Weisbecker <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Waiman-Long

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

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;
 		}