linux-kernel - Re: [RFC/RFT][PATCH v3 5/6] sched: idle: Select idle state before stopping the tick

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Date:   Sun, 11 Mar 2018 02:44:37 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/RFT][PATCH v3 5/6] sched: idle: Select idle state before
 stopping the tick

On Fri, Mar 09, 2018 at 10:46:55AM +0100, Rafael J. Wysocki wrote:
> --- linux-pm.orig/kernel/time/tick-sched.h
> +++ linux-pm/kernel/time/tick-sched.h
> @@ -30,6 +30,7 @@ enum tick_nohz_mode {
>   *			when the CPU returns from nohz sleep.
>   * @next_tick:		Next tick to be fired when in dynticks mode.
>   * @tick_stopped:	Indicator that the idle tick has been stopped
> + * @tick_may_stop:	Indicator that the idle tick may be stopped shortly

Perhaps we can set timer_expires to 0 instead when we want to invalidate
the last value?

>   * @idle_jiffies:	jiffies at the entry to idle for idle time accounting
>   * @idle_calls:		Total number of idle calls
>   * @idle_sleeps:	Number of idle calls, where the sched tick was stopped
> @@ -38,7 +39,6 @@ enum tick_nohz_mode {
>   * @idle_exittime:	Time when the idle state was left
>   * @idle_sleeptime:	Sum of the time slept in idle with sched tick stopped
>   * @iowait_sleeptime:	Sum of the time slept in idle with sched tick stopped, with IO outstanding
> - * @sleep_length:	Duration of the current idle sleep
>   * @do_timer_lst:	CPU was the last one doing do_timer before going idle
>   */
>  struct tick_sched {
> @@ -49,6 +49,7 @@ struct tick_sched {
>  	ktime_t				next_tick;
>  	int				inidle;
>  	int				tick_stopped;
> +	int				tick_may_stop;
>  	unsigned long			idle_jiffies;
>  	unsigned long			idle_calls;
>  	unsigned long			idle_sleeps;
> @@ -58,8 +59,9 @@ struct tick_sched {
>  	ktime_t				idle_exittime;
>  	ktime_t				idle_sleeptime;
>  	ktime_t				iowait_sleeptime;
> -	ktime_t				sleep_length;
>  	unsigned long			last_jiffies;
> +	u64				timer_expires;
> +	u64				timer_expires_basemono;

We may need documentation for the above fields too.

> Index: linux-pm/kernel/time/tick-sched.c
> ===================================================================
> --- linux-pm.orig/kernel/time/tick-sched.c
> +++ linux-pm/kernel/time/tick-sched.c
> @@ -652,13 +652,10 @@ static inline bool local_timer_softirq_p
>  	return local_softirq_pending() & TIMER_SOFTIRQ;
>  }
>  
> -static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> -					 ktime_t now, int cpu)
> +static ktime_t __tick_nohz_next_event(struct tick_sched *ts, int cpu)

Since we don't seem to have a lower level version, can we remove the underscores?

>  {
> -	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
>  	u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
>  	unsigned long seq, basejiff;
> -	ktime_t	tick;
>  
>  	/* Read jiffies and the time when jiffies were updated last */
>  	do {
[...]


> +static void __tick_nohz_stop_tick(struct tick_sched *ts, int cpu)

(Same comment here about the underscores).

> +{
> +	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
> +	u64 basemono = ts->timer_expires_basemono;
> +	u64 expires = ts->timer_expires;
> +	ktime_t tick = expires;
> +
> +	/* Make sure we won't be trying to stop it twice in a row. */
> +	ts->tick_may_stop = 0;
> +
> +	/*
> +	 * If this CPU is the one which updates jiffies, then give up
> +	 * the assignment and let it be taken by the CPU which runs
> +	 * the tick timer next, which might be this CPU as well. If we
> +	 * don't drop this here the jiffies might be stale and
> +	 * do_timer() never invoked. Keep track of the fact that it
> +	 * was the one which had the do_timer() duty last.
> +	 */
> +	if (cpu == tick_do_timer_cpu) {
> +		tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> +		ts->do_timer_last = 1;
> +	}
>  
>  	/* Skip reprogram of event if its not changed */
>  	if (ts->tick_stopped && (expires == ts->next_tick)) {
>  		/* Sanity check: make sure clockevent is actually programmed */
>  		if (tick == KTIME_MAX || ts->next_tick == hrtimer_get_expires(&ts->sched_timer))
> -			goto out;
> +			return;
>  
>  		WARN_ON_ONCE(1);
>  		printk_once("basemono: %llu ts->next_tick: %llu dev->next_event: %llu timer->active: %d timer->expires: %llu\n",
[...]
> +void tick_nohz_idle_retain_tick(void)
> +{
> +	__this_cpu_write(tick_cpu_sched.tick_may_stop, 0);
> +}
> +

So, I've become overly-paranoid about cached expiration on nohz code, we've run
into bugs that took months to debug before. It seems that the cached version shouldn't
leak in any way there, still can we have checks such as this in tick_nohz_idle_enter/exit()?

     WARN_ON_ONCE(__this_cpu_read(tick_cpu_sched.tick_may_stop));

Otherwise a leaking cached expiration may mislead further nohz tick stop and
bypass calls to tick_nohz_next_event().

Also let's make sure we never call tick_nohz_get_sleep_length() outside idle:

     WARN_ON_ONCE(!__this_cpu_read(tick_cpu_sched.inidle));

Thanks!