linux-kernel - [PATCH v2] tick/nohz_full: turn tick_do_timer_boot_cpu into boot_cpu_is_nohz

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Date: Mon, 3 Jun 2024 17:35:57 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Nicholas Piggin <npiggin@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, Phil Auld <pauld@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc: Chris von Recklinghausen <crecklin@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] tick/nohz_full: turn tick_do_timer_boot_cpu into
 boot_cpu_is_nohz_full

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@...hat.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