Re: [PATCH v2] PM: s2idle: Make sure CPUs will wakeup directly on resume

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 08, 2024 at 09:02:23AM +0200, Anna-Maria Behnsen wrote:
> s2idle works like a regular suspend with freezing processes and freezing
> devices. All CPUs except the control CPU go into idle. Once this is
> completed the control CPU kicks all other CPUs out of idle, so that they
> reenter the idle loop and then enter s2idle state. The control CPU then
> issues an swait() on the suspend state and therefore enters the idle loop
> as well.
> 
> Due to being kicked out of idle, the other CPUs leave their NOHZ states,
> which means the tick is active and the corresponding hrtimer is programmed
> to the next jiffie.
> 
> On entering s2idle the CPUs shut down their local clockevent device to
> prevent wakeups. The last CPU which enters s2idle shuts down its local
> clockevent and freezes timekeeping.
> 
> On resume, one of the CPUs receives the wakeup interrupt, unfreezes
> timekeeping and its local clockevent and starts the resume process. At that
> point all other CPUs are still in s2idle with their clockevents switched
> off. They only resume when they are kicked by another CPU or after resuming
> devices and then receiving a device interrupt.
> 
> That means there is no guarantee that all CPUs will wakeup directly on
> resume. As a consequence there is no guarantee that timers which are queued
> on those CPUs and should expire directly after resume, are handled. Also
> timer list timers which are remotely queued to one of those CPUs after
> resume will not result in a reprogramming IPI as the tick is
> active. Queueing a hrtimer will also not result in a reprogramming IPI
> because the first hrtimer event is already in the past.
> 
> The recent introduction of the timer pull model (7ee988770326 ("timers:
> Implement the hierarchical pull model")) amplifies this problem, if the
> current migrator is one of the non woken up CPUs. When a non pinned timer
> list timer is queued and the queuing CPU goes idle, it relies on the still
> suspended migrator CPU to expire the timer which will happen by chance.
> 
> The problem exists since commit 8d89835b0467 ("PM: suspend: Do not pause
> cpuidle in the suspend-to-idle path"). There the cpuidle_pause() call which
> in turn invoked a wakeup for all idle CPUs was moved to a later point in
> the resume process. This might not be reached or reached very late because
> it waits on a timer of a still suspended CPU.
> 
> Address this by kicking all CPUs out of idle after the control CPU returns
> from swait() so that they resume their timers and restore consistent system
> state.
> 
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218641
> Fixes: 8d89835b0467 ("PM: suspend: Do not pause cpuidle in the suspend-to-idle path")
> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> Tested-by: Mario Limonciello <mario.limonciello@amd.com>
> Cc: stable@kernel.org

Cute,

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
>  kernel/power/suspend.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> --- a/kernel/power/suspend.c
> +++ b/kernel/power/suspend.c
> @@ -106,6 +106,12 @@ static void s2idle_enter(void)
>  	swait_event_exclusive(s2idle_wait_head,
>  		    s2idle_state == S2IDLE_STATE_WAKE);
>  
> +	/*
> +	 * Kick all CPUs to ensure that they resume their timers and restore
> +	 * consistent system state.
> +	 */
> +	wake_up_all_idle_cpus();
> +
>  	cpus_read_unlock();
>  
>  	raw_spin_lock_irq(&s2idle_lock);