[PATCH v2] tick/nohz_full: turn tick_do_timer_boot_cpu into boot_cpu_is_nohz_full

Oleg Nesterov
2024-06-03T15:37:34Z

We don't need to record the cpu number of the nohz_full boot CPU, a simple
boolean is enough. We could even kill it, the "else if" branch could check
tick_nohz_full_cpu(tick_do_timer_cpu) && !tick_nohz_full_cpu(cpu).

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/time/tick-common.c | 21 ++++-----------------
1 file changed, 4 insertions(+), 17 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 27d0018c8b05..82c1b2206631 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -49,15 +49,6 @@ ktime_t tick_next_period;
* procedure also covers cpu hotplug.
*/
int tick_do_timer_cpu __read_mostly = TICK_DO_TIMER_BOOT;
-#ifdef CONFIG_NO_HZ_FULL
-/*
- * tick_do_timer_boot_cpu indicates the boot CPU temporarily owns
- * tick_do_timer_cpu and it should be taken over by an eligible secondary
- * when one comes online.
- */
-static int tick_do_timer_boot_cpu __read_mostly = -1;
-#endif
-
/*
* Debugging: see timer_list.c
*/
@@ -192,6 +183,7 @@ static void tick_setup_device(struct tick_device *td,
* First device setup ?
*/
if (!td->evtdev) {
+ static bool boot_cpu_is_nohz_full __read_mostly;
/*
* If no cpu took the do_timer update, assign it to
* this cpu:
@@ -199,18 +191,14 @@ static void tick_setup_device(struct tick_device *td,
if (READ_ONCE(tick_do_timer_cpu) == TICK_DO_TIMER_BOOT) {
WRITE_ONCE(tick_do_timer_cpu, cpu);
tick_next_period = ktime_get();
-#ifdef CONFIG_NO_HZ_FULL
/*
* The boot CPU may be nohz_full, in which case the
* first housekeeping secondary will take do_timer()
* from us.
*/
- if (tick_nohz_full_cpu(cpu))
- tick_do_timer_boot_cpu = cpu;
-
- } else if (tick_do_timer_boot_cpu != -1 &&
- !tick_nohz_full_cpu(cpu)) {
- tick_do_timer_boot_cpu = -1;
+ boot_cpu_is_nohz_full = tick_nohz_full_cpu(cpu);
+ } else if (boot_cpu_is_nohz_full && !tick_nohz_full_cpu(cpu)) {
+ boot_cpu_is_nohz_full = false;
/*
* The boot CPU will stay in periodic (NOHZ disabled)
* mode until clocksource_done_booting() called after
@@ -221,7 +209,6 @@ static void tick_setup_device(struct tick_device *td,
* check in tick_periodic() but this race is harmless.
*/
WRITE_ONCE(tick_do_timer_cpu, cpu);
-#endif
}

/*
--
2.25.1.362.g51ebf55