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.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240225225508.11587-12-frederic@kernel.org

diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index 35808bbb8a47875d9b785a56fddeaa7967a7b867..3b555e0fa9378380212f2533351ab1434ac97703 100644 (file)
--- 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;