Subject: [PATCH 11/15] tick/sched: Account tickless idle cputime only when tick is stopped

From: Frederic Weisbecker <[email-masked]>
Date: Fri, 6 Feb 2026 15:22:41 +0100

There is no real point in switching to dyntick-idle cputime accounting
mode if the tick is not actually stopped. This just adds overhead,
notably fetching the GTOD, on each idle exit and each idle IRQ entry for
no reason during short idle trips.

Signed-off-by: Frederic Weisbecker <[email-masked]>
---
 kernel/time/tick-sched.c | 44 ++++++++++++++++++----------------------
 1 file changed, 20 insertions(+), 24 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index aa36c8d218e2..bceed0c4dd2c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1141,8 +1141,10 @@ void tick_nohz_idle_stop_tick(void)
 		ts->idle_sleeps++;
 		ts->idle_expires = expires;
 
-		if (!was_stopped && tick_sched_flag_test(ts, TS_FLAG_STOPPED))
+		if (!was_stopped && tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
+			kcpustat_dyntick_start(ts->idle_entrytime);
 			nohz_balance_enter_idle(cpu);
+		}
 	} else {
 		tick_nohz_retain_tick(ts);
 	}
@@ -1184,7 +1186,6 @@ void tick_nohz_idle_enter(void)
 	WARN_ON_ONCE(ts->timer_expires_base);
 	tick_sched_flag_set(ts, TS_FLAG_INIDLE);
 	ts->idle_entrytime = ktime_get();
-	kcpustat_dyntick_start(ts->idle_entrytime);
 	tick_nohz_clock_sleep(ts);
 
 	local_irq_enable();
@@ -1214,9 +1215,10 @@ void tick_nohz_irq_exit(void)
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
 
 	if (tick_sched_flag_test(ts, TS_FLAG_INIDLE)) {
-		ts->idle_entrytime = ktime_get();
-		kcpustat_irq_exit(ts->idle_entrytime);
 		tick_nohz_clock_sleep(ts);
+		ts->idle_entrytime = ktime_get();
+		if (tick_sched_flag_test(ts, TS_FLAG_STOPPED))
+			kcpustat_irq_exit(ts->idle_entrytime);
 	} else {
 		tick_nohz_full_update_tick(ts);
 	}
@@ -1317,8 +1319,11 @@ void tick_nohz_idle_restart_tick(void)
 {
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
 
-	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED))
-		tick_nohz_restart_sched_tick(ts, ktime_get());
+	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
+		ktime_t now = ktime_get();
+		kcpustat_dyntick_stop(now);
+		tick_nohz_restart_sched_tick(ts, now);
+	}
 }
 
 static void tick_nohz_idle_update_tick(struct tick_sched *ts, ktime_t now)
@@ -1348,7 +1353,6 @@ static void tick_nohz_idle_update_tick(struct tick_sched *ts, ktime_t now)
 void tick_nohz_idle_exit(void)
 {
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
-	bool idle_active, tick_stopped;
 	ktime_t now;
 
 	local_irq_disable();
@@ -1357,18 +1361,13 @@ void tick_nohz_idle_exit(void)
 	WARN_ON_ONCE(ts->timer_expires_base);
 
 	tick_sched_flag_clear(ts, TS_FLAG_INIDLE);
-	idle_active = tick_sched_flag_test(ts, TS_FLAG_IDLE_ACTIVE);
-	tick_stopped = tick_sched_flag_test(ts, TS_FLAG_STOPPED);
+	tick_nohz_clock_wakeup(ts);
 
-	if (idle_active || tick_stopped)
+	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
 		now = ktime_get();
-
-	if (idle_active)
-		tick_nohz_clock_wakeup(ts);
-
-	if (tick_stopped)
+		kcpustat_dyntick_stop(now);
 		tick_nohz_idle_update_tick(ts, now);
-	kcpustat_dyntick_stop(now);
+	}
 
 	local_irq_enable();
 }
@@ -1422,15 +1421,13 @@ static inline void tick_nohz_irq_enter(void)
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
 	ktime_t now;
 
-	if (!tick_sched_flag_test(ts, TS_FLAG_STOPPED | TS_FLAG_IDLE_ACTIVE))
+	tick_nohz_clock_wakeup(ts);
+
+	if (!tick_sched_flag_test(ts, TS_FLAG_STOPPED))
 		return;
 
 	now = ktime_get();
-
-	if (tick_sched_flag_test(ts, TS_FLAG_IDLE_ACTIVE)) {
-		tick_nohz_clock_wakeup(ts);
-		kcpustat_irq_enter(now);
-	}
+	kcpustat_irq_enter(now);
 
 	/*
 	 * If all CPUs are idle we may need to update a stale jiffies value.
@@ -1439,8 +1436,7 @@ static inline void tick_nohz_irq_enter(void)
 	 * rare case (typically stop machine). So we must make sure we have a
 	 * last resort.
 	 */
-	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED))
-		tick_nohz_update_jiffies(now);
+	tick_nohz_update_jiffies(now);
 }
 
 #else
-- 
2.51.1