tick/sched: Account tickless idle cputime only when tick is stopped

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 <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://patch.msgid.link/20260508131647.43868-12-frederic@kernel.org

Author: Frederic Weisbecker

kernel/time/tick-sched.c

@@ -1157,8 +1157,10 @@ 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);
 	}
@@ -1200,7 +1202,6 @@ 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();
@@ -1230,9 +1231,10 @@ 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);
 	}
@@ -1333,8 +1335,17 @@ 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)) {
+		/*
+		 * Update entrytime here in case the tick restart is due to temporary
+		 * polling on forced broadcast. The tick may be stopped again later within
+		 * the same idle trip. The idle_entrytime was updated recently but make sure
+		 * no tiny amount of idle time is accounted twice.
+		 */
+		ts->idle_entrytime = ktime_get();
+		kcpustat_dyntick_stop(ts->idle_entrytime);
+		tick_nohz_restart_sched_tick(ts, ts->idle_entrytime);
+	}
 }
 
 static void tick_nohz_idle_update_tick(struct tick_sched *ts, ktime_t now)
@@ -1364,7 +1375,6 @@ 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();
@@ -1373,18 +1383,13 @@ 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();
 }
@@ -1439,15 +1444,13 @@ 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.
@@ -1456,8 +1459,7 @@ 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