linux-kernel - [PATCH 11/15] tick: Move got_idle

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Message-ID: <20240131231120.12006-12-frederic@kernel.org>
Date: Thu,  1 Feb 2024 00:11:16 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Peng Liu <liupeng17@...ovo.com>,
	Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 11/15] tick: Move got_idle_tick away from common flags

tick_nohz_idle_got_tick() is called by cpuidle_reflect() within the idle
loop with interrupts enabled. This function modifies the struct
tick_sched's bitfield "got_idle_tick". However this bitfield is stored
within the same mask as other bitfields that can be modified from
interrupts.

Fortunately so far it looks like the only race that can happen is while
writing ->got_idle_tick to 0, an interrupt fires and writes the
->idle_active field to 0. It's then possible that the interrupted write
to ->got_idle_tick writes back the old value of ->idle_active back to 1.

However if that happens, the worst possible outcome is that the time
spent between that interrupt and the upcoming call to
tick_nohz_idle_exit() is accounted as idle, which is negligible quantity.

Still all the bitfield writes within this struct tick_sched's shadow
mask should be IRQ-safe. Therefore move this bitfield out to its own
storage to avoid further suprises.

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/time/tick-sched.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index 35808bbb8a47..3b555e0fa937 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -61,7 +61,6 @@ struct tick_sched {
 	unsigned int			tick_stopped	: 1;
 	unsigned int			idle_active	: 1;
 	unsigned int			do_timer_last	: 1;
-	unsigned int			got_idle_tick	: 1;
 
 	/* Tick handling: jiffies stall check */
 	unsigned int			stalled_jiffies;
@@ -73,6 +72,7 @@ struct tick_sched {
 	ktime_t				next_tick;
 	unsigned long			idle_jiffies;
 	ktime_t				idle_waketime;
+	unsigned int			got_idle_tick;
 
 	/* Idle entry */
 	seqcount_t			idle_sleeptime_seq;
-- 
2.43.0