Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

[Marcelo Tosatti]

On Fri, Apr 22, 2022 at 08:36:47PM +0100, Aaron Tomlin wrote:
> Hi Frederic and Marcelo,
> 
> Oops, please ignore RFC Patch v2 [1] since I forgot to compile test - sorry
> about that! Any feedback would be appreciated. Thanks.
> 
> [1]: https://lore.kernel.org/lkml/20220422112053.3695526-1-atomlin@xxxxxxxxxx/
> 
> 
> 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.
> 
> A customer provided some evidence which indicates that the idle tick was
> stopped; albeit, CPU-specific vmstat counters still remained populated.
> Thus one can only assume quiet_vmstat() was not invoked on return to the
> idle loop.
> 
> If I understand correctly, I suspect this divergence might erroneously
> prevent a reclaim attempt by kswapd. If the number of zone specific free
> pages are below their per-cpu drift value then
> zone_page_state_snapshot() is used to compute a more accurate view of
> the aforementioned statistic.  Thus any task blocked on the NUMA node
> specific pfmemalloc_wait queue will be unable to make significant
> progress via direct reclaim unless it is killed after being woken up by
> kswapd (see throttle_direct_reclaim()).
> 
> Consider the following theoretical scenario:
> 
>         1.      CPU Y migrated running task A to CPU X that was
>                 in an idle state i.e. waiting for an IRQ - not
>                 polling; marked the current task on CPU X to
>                 need/or require a reschedule i.e., set
>                 TIF_NEED_RESCHED and invoked a reschedule IPI to
>                 CPU X (see sched_move_task())
> 
>         2.      CPU X acknowledged the reschedule IPI from CPU Y;
>                 generic idle loop code noticed the
>                 TIF_NEED_RESCHED flag against the idle task and
>                 attempts to exit of the loop and calls the main
>                 scheduler function i.e. __schedule().
> 
>                 Since the idle tick was previously stopped no
>                 scheduling-clock tick would occur.
>                 So, no deferred timers would be handled
> 
>         3.      Post transition to kernel execution Task A
>                 running on CPU Y, indirectly released a few pages
>                 (e.g. see __free_one_page()); CPU Y's
>                 'vm_stat_diff[NR_FREE_PAGES]' was updated and zone
>                 specific 'vm_stat[]' update was deferred as per the
>                 CPU-specific stat threshold
> 
>         4.      Task A does invoke exit(2) and the kernel does
>                 remove the task from the run-queue; the idle task
>                 was selected to execute next since there are no
>                 other runnable tasks assigned to the given CPU
>                 (see pick_next_task() and pick_next_task_idle())
> 
>         5.      On return to the idle loop since the idle tick
>                 was already stopped and can remain so (see [1]
>                 below) e.g. no pending soft IRQs, no attempt is
>                 made to zero and fold CPU Y's vmstat counters
>                 since reprogramming of the scheduling-clock tick
>                 is not required/or needed (see [2])
> 
> 		  ...
> 		    do_idle
> 		    {
> 
> 		      __current_set_polling()
> 		      tick_nohz_idle_enter()
> 
> 		      while (!need_resched()) {
> 
> 			local_irq_disable()
> 
> 			...
> 
> 			/* No polling or broadcast event */
> 			cpuidle_idle_call()
> 			{
> 
> 			  if (cpuidle_not_available(drv, dev)) {
> 			    tick_nohz_idle_stop_tick()
> 			      __tick_nohz_idle_stop_tick(this_cpu_ptr(&tick_cpu_sched))
> 			      {
> 				int cpu = smp_processor_id()
> 
> 				if (ts->timer_expires_base)
> 				  expires = ts->timer_expires
> 				else if (can_stop_idle_tick(cpu, ts))
> 	      (1) ------->        expires = tick_nohz_next_event(ts, cpu)
> 				else
> 				  return
> 
> 				ts->idle_calls++
> 
> 				if (expires > 0LL) {
> 
> 				  tick_nohz_stop_tick(ts, cpu)
> 				  {
> 
> 				    if (ts->tick_stopped && (expires == ts->next_tick)) {
> 	      (2) ------->            if (tick == KTIME_MAX || ts->next_tick ==
> 					hrtimer_get_expires(&ts->sched_timer))
> 					return
> 				    }
> 				    ...
> 				  }
> 
> So the idea of with this patch is to ensure refresh_cpu_vm_stats(false) is
> called, when it is appropriate, on return to the idle loop when the idle
> tick was previously stopped too. Additionally, when the scheduling-tick
> is stopped and a task in kernel-mode, modifies the CPU-specific
> 'vm_stat_diff[]' and goes to user-mode for a long time.
> 
> Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxx>
> ---
>  include/linux/tick.h     |  9 ++-------
>  kernel/time/tick-sched.c | 18 +++++++++++++++++-
>  2 files changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index bfd571f18cfd..4c576c9ca0a2 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -11,7 +11,6 @@
>  #include <linux/context_tracking_state.h>
>  #include <linux/cpumask.h>
>  #include <linux/sched.h>
> -#include <linux/rcupdate.h>
>  
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
>  extern void __init tick_init(void);
> @@ -123,6 +122,8 @@ enum tick_dep_bits {
>  #define TICK_DEP_MASK_RCU		(1 << TICK_DEP_BIT_RCU)
>  #define TICK_DEP_MASK_RCU_EXP		(1 << TICK_DEP_BIT_RCU_EXP)
>  
> +void tick_nohz_user_enter_prepare(void);
> +
>  #ifdef CONFIG_NO_HZ_COMMON
>  extern bool tick_nohz_enabled;
>  extern bool tick_nohz_tick_stopped(void);
> @@ -305,10 +306,4 @@ static inline void tick_nohz_task_switch(void)
>  		__tick_nohz_task_switch();
>  }
>  
> -static inline void tick_nohz_user_enter_prepare(void)
> -{
> -	if (tick_nohz_full_cpu(smp_processor_id()))
> -		rcu_nocb_flush_deferred_wakeup();
> -}
> -
>  #endif
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index d257721c68b8..c6cac2d8e8ed 100644
> --- a/kernel/time/tick-sched.c
> +++ b/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>
>  
> @@ -43,6 +44,19 @@ struct tick_sched *tick_get_tick_sched(int cpu)
>  	return &per_cpu(tick_cpu_sched, cpu);
>  }
>  
> +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();
> +	}
> +}

So you are syncing the vmstats on every system call return:

static void exit_to_user_mode_prepare(struct pt_regs *regs)
{
        unsigned long ti_work = read_thread_flags();
        
        lockdep_assert_irqs_disabled();

        /* Flush pending rcuog wakeup before the last need_resched() check */
        tick_nohz_user_enter_prepare();

As PeterZ mentioned for idle (which can be very frequent), returning 
to userspace can be very frequent as well. 

Have you measured performance of any system call heavy application
with this change?

>From my experience with the task isolation patchset, it will cause
noticeable slowdown to system call return.

Then the comment on why its so slow:

"This loop is quite heavy. Maybe reducing the data necessary to be read
to a couple of cachelines would improve it considerably."

The comment:

"Is there anything that prevents a nohz full CPU from running an
application with short and frequent idling?"

Is confusing and can be ignored.

> +
>  #if defined(CONFIG_NO_HZ_COMMON) || defined(CONFIG_HIGH_RES_TIMERS)
>  /*
>   * The time, when the last jiffy update happened. Write access must hold
> @@ -891,6 +905,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>  		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 */
> @@ -912,7 +929,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
>  	 */
>  	if (!ts->tick_stopped) {
>  		calc_load_nohz_start();
> -		quiet_vmstat();
>  
>  		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
>  		ts->tick_stopped = 1;
> -- 
> 2.34.1
> 
>