Re: [PATCH v7 2/3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 17 Aug 2022 16:13:48 -0300 Marcelo Tosatti <mtosatti@redhat.com> wrote:

> From: Aaron Tomlin <atomlin@redhat.com>
> 
> In the context of the idle task and an adaptive-tick mode/or a nohz_full
> CPU, quiet_vmstat() can be called: before stopping the idle tick,
> entering an idle state and on exit. In particular, for the latter case,
> when the idle task is required to reschedule, the idle tick can remain
> stopped and the timer expiration time endless i.e., KTIME_MAX. Now,
> indeed before a nohz_full CPU enters an idle state, CPU-specific vmstat
> counters should be processed to ensure the respective values have been
> reset and folded into the zone specific 'vm_stat[]'. That being said, it
> can only occur when: the idle tick was previously stopped, and
> reprogramming of the timer is not required.

I'd like to see input from tick/sched maintainers before toughing this
one, please.

> --- linux-2.6.orig/kernel/time/tick-sched.c
> +++ linux-2.6/kernel/time/tick-sched.c
> @@ -26,6 +26,7 @@
>  #include <linux/posix-timers.h>
>  #include <linux/context_tracking.h>
>  #include <linux/mm.h>
> +#include <linux/rcupdate.h>
>  
>  #include <asm/irq_regs.h>
>  
> @@ -519,6 +520,20 @@ void __tick_nohz_task_switch(void)
>  	}
>  }
>  
> +void __tick_nohz_user_enter_prepare(void)
> +{
> +	struct tick_sched *ts;
> +
> +	if (tick_nohz_full_cpu(smp_processor_id())) {
> +		ts = this_cpu_ptr(&tick_cpu_sched);
> +
> +		if (ts->tick_stopped)
> +			quiet_vmstat();
> +		rcu_nocb_flush_deferred_wakeup();
> +	}
> +}
> +EXPORT_SYMBOL_GPL(__tick_nohz_user_enter_prepare);
> +
>  /* Get the boot-time nohz CPU list from the kernel parameters. */
>  void __init tick_nohz_full_setup(cpumask_var_t cpumask)
>  {
> @@ -890,6 +905,9 @@ static void tick_nohz_stop_tick(struct t
>  		ts->do_timer_last = 0;
>  	}
>  
> +	/* Attempt to fold when the idle tick is stopped or not */
> +	quiet_vmstat();
> +
>  	/* Skip reprogram of event if its not changed */
>  	if (ts->tick_stopped && (expires == ts->next_tick)) {
>  		/* Sanity check: make sure clockevent is actually programmed */
> @@ -911,7 +929,6 @@ static void tick_nohz_stop_tick(struct t
>  	 */
>  	if (!ts->tick_stopped) {
>  		calc_load_nohz_start();
> -		quiet_vmstat();
>  
>  		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
>  		ts->tick_stopped = 1;

Putting vmstat stuff inside core timer code is unattractive, to say the
least!