Re: [PATCH] PM / suspend: Count suspend-to-idle loop as sleep time

["Mika Penttilä" <mika.penttila@nextfour.com>]

Hi!


On 09/14/2018 09:59 AM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> There is a difference in behavior between suspend-to-idle and
> suspend-to-RAM in the timekeeping handling that leads to functional
> issues.  Namely, every iteration of the loop in s2idle_loop()
> increases the monotinic clock somewhat, even if timekeeping_suspend()
> and timekeeping_resume() are invoked from s2idle_enter(), and if
> many of them are carried out in a row, the monotonic clock can grow
> significantly while the system is regarded as suspended, which
> doesn't happen during suspend-to-RAM and so it is unexpected and
> leads to confusion and misbehavior in user space (similar to what
> ensued when we tried to combine the boottime and monotonic clocks).
> 
> To avoid that, count all iterations of the loop in s2idle_loop()
> as "sleep time" and adjust the clock for that on exit from
> suspend-to-idle.
> 
> [That also covers systems on which timekeeping is not suspended
>  by by s2idle_enter().]
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> 
> This is a replacement for https://patchwork.kernel.org/patch/10599209/
> 
> I decided to count the entire loop in s2idle_loop() as "sleep time" as the
> patch is then simpler and it also covers systems where timekeeping is not
> suspended in the final step of suspend-to-idle.
> 
> I dropped the "Fixes:" tag, because the monotonic clock delta problem
> has been present on the latter since the very introduction of "freeze"
> (as suspend-to-idle was referred to previously) and so this doesn't fix
> any particular later commits.
> 
> ---
>  kernel/power/suspend.c |   18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> Index: linux-pm/kernel/power/suspend.c
> ===================================================================
> --- linux-pm.orig/kernel/power/suspend.c
> +++ linux-pm/kernel/power/suspend.c
> @@ -109,8 +109,12 @@ static void s2idle_enter(void)
>  
>  static void s2idle_loop(void)
>  {
> +	ktime_t start, delta;
> +
>  	pm_pr_dbg("suspend-to-idle\n");
>  
> +	start = ktime_get();
> +
>  	for (;;) {
>  		int error;
>  
> @@ -150,6 +154,20 @@ static void s2idle_loop(void)
>  		pm_wakeup_clear(false);
>  	}
>  
> +	/*
> +	 * If the monotonic clock difference between the start of the loop and
> +	 * this point is too large, user space may get confused about whether or
> +	 * not the system has been suspended and tasks may get killed by
> +	 * watchdogs etc., so count the loop as "sleep time" to compensate for
> +	 * that.
> +	 */
> +	delta = ktime_sub(ktime_get(), start);
> +	if (ktime_to_ns(delta) > 0) {
> +		struct timespec64 timespec64_delta = ktime_to_timespec64(delta);
> +
> +		timekeeping_inject_sleeptime64(&timespec64_delta);
> +	}

But doesn't injecting sleep time here make monotonic clock too large by the amount of sleeptime? 
tick_freeze() / tick_unfreeze() already injects the sleeptime (otherwise delta would be 0).

> +
>  	pm_pr_dbg("resume from suspend-to-idle\n");
>  }
>  
>