[RFC PATCH v4] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

[Aaron Tomlin]

Hi Frederic and Marcelo,

I have incorporated an idea from Marcelo's patch [1] where a CPU-specific
variable is used to indicate if a vmstat differential/or imbalance is
present for a given CPU. So, at the appropriate time, vmstat processing can
be initiated. The hope is that this particular approach is "cheaper" when
compared to need_update() - used currently; in the context of nohz_full and
the scheduling-clock tick being stopped, we would now with this patch,
check if a CPU-specific vmstat imbalance is present before exiting
user-mode (see tick_nohz_user_enter_prepare()).

This trivial test program [2] was used to determine the somewhat impact
under vanilla and with the proposed changes; mlock(2) and munlock(2) was
used solely to modify vmstat item 'NR_MLOCK'. The following is an average
count of CPU-cycles across the aforementioned system calls and the idle
loop, respectively. I believe these results are negligible:

	  Modified		   |  		Vanilla
                                   |
                                   |
  cycles per syscall: 7399         | 	cycles per syscall: 4150
  cycles per idle loop: 141048     |	cycles per idle loop: 144730
                                   |


Any feedback would be appreciated. Thanks.

Changes since v3 [3]:

 - Used EXPORT_SYMBOL() on tick_nohz_user_enter_prepare()
 - Replaced need_update()
 - Introduced CPU-specific variable namely 'vmstat_dirty'
   and mark_vmstat_dirty()

[1]: https://lore.kernel.org/lkml/20220204173554.763888172@fedora.localdomain/
[2]: https://pastebin.com/8AtzSAuK
[3]: https://lore.kernel.org/lkml/20220422193647.3808657-1-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, in the context of
nohz_full, when the scheduling-tick is stopped and before exiting
to user-mode, ensure no CPU-specific vmstat differentials remain.

Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxx>
---
 include/linux/tick.h     |  9 ++------
 kernel/time/tick-sched.c | 19 ++++++++++++++++-
 mm/vmstat.c              | 46 +++++++++++++---------------------------
 3 files changed, 35 insertions(+), 39 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..4cdd71cc292f 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 +44,20 @@ 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();
+	}
+}
+EXPORT_SYMBOL(tick_nohz_user_enter_prepare);
+
 #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 +906,9 @@ 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 +930,6 @@ 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;
diff --git a/mm/vmstat.c b/mm/vmstat.c
index b75b1a64b54c..7bfcafafe8f7 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -195,6 +195,12 @@ void fold_vm_numa_events(void)
 #endif
 
 #ifdef CONFIG_SMP
+static DEFINE_PER_CPU_ALIGNED(bool, vmstat_dirty);
+
+static inline void mark_vmstat_dirty(void)
+{
+	this_cpu_write(vmstat_dirty, true);
+}
 
 int calculate_pressure_threshold(struct zone *zone)
 {
@@ -367,6 +373,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 		x = 0;
 	}
 	__this_cpu_write(*p, x);
+	mark_vmstat_dirty();
 
 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
 		preempt_enable();
@@ -405,6 +412,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
 		x = 0;
 	}
 	__this_cpu_write(*p, x);
+	mark_vmstat_dirty();
 
 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
 		preempt_enable();
@@ -603,6 +611,7 @@ static inline void mod_zone_state(struct zone *zone,
 
 	if (z)
 		zone_page_state_add(z, zone, item);
+	mark_vmstat_dirty();
 }
 
 void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
@@ -671,6 +680,7 @@ static inline void mod_node_state(struct pglist_data *pgdat,
 
 	if (z)
 		node_page_state_add(z, pgdat, item);
+	mark_vmstat_dirty();
 }
 
 void mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
@@ -1866,6 +1876,7 @@ int sysctl_stat_interval __read_mostly = HZ;
 static void refresh_vm_stats(struct work_struct *work)
 {
 	refresh_cpu_vm_stats(true);
+	this_cpu_write(vmstat_dirty, false);
 }
 
 int vmstat_refresh(struct ctl_table *table, int write,
@@ -1930,6 +1941,7 @@ int vmstat_refresh(struct ctl_table *table, int write,
 static void vmstat_update(struct work_struct *w)
 {
 	if (refresh_cpu_vm_stats(true)) {
+		this_cpu_write(vmstat_dirty, false);
 		/*
 		 * Counters were updated so we expect more updates
 		 * to occur in the future. Keep on running the
@@ -1941,35 +1953,6 @@ static void vmstat_update(struct work_struct *w)
 	}
 }
 
-/*
- * Check if the diffs for a certain cpu indicate that
- * an update is needed.
- */
-static bool need_update(int cpu)
-{
-	pg_data_t *last_pgdat = NULL;
-	struct zone *zone;
-
-	for_each_populated_zone(zone) {
-		struct per_cpu_zonestat *pzstats = per_cpu_ptr(zone->per_cpu_zonestats, cpu);
-		struct per_cpu_nodestat *n;
-
-		/*
-		 * The fast way of checking if there are any vmstat diffs.
-		 */
-		if (memchr_inv(pzstats->vm_stat_diff, 0, sizeof(pzstats->vm_stat_diff)))
-			return true;
-
-		if (last_pgdat == zone->zone_pgdat)
-			continue;
-		last_pgdat = zone->zone_pgdat;
-		n = per_cpu_ptr(zone->zone_pgdat->per_cpu_nodestats, cpu);
-		if (memchr_inv(n->vm_node_stat_diff, 0, sizeof(n->vm_node_stat_diff)))
-			return true;
-	}
-	return false;
-}
-
 /*
  * Switch off vmstat processing and then fold all the remaining differentials
  * until the diffs stay at zero. The function is used by NOHZ and can only be
@@ -1983,7 +1966,7 @@ void quiet_vmstat(void)
 	if (!delayed_work_pending(this_cpu_ptr(&vmstat_work)))
 		return;
 
-	if (!need_update(smp_processor_id()))
+	if (!__this_cpu_read(vmstat_dirty))
 		return;
 
 	/*
@@ -1993,6 +1976,7 @@ void quiet_vmstat(void)
 	 * vmstat_shepherd will take care about that for us.
 	 */
 	refresh_cpu_vm_stats(false);
+	__this_cpu_write(vmstat_dirty, false);
 }
 
 /*
@@ -2014,7 +1998,7 @@ static void vmstat_shepherd(struct work_struct *w)
 	for_each_online_cpu(cpu) {
 		struct delayed_work *dw = &per_cpu(vmstat_work, cpu);
 
-		if (!delayed_work_pending(dw) && need_update(cpu))
+		if (!delayed_work_pending(dw) && per_cpu(vmstat_dirty, cpu))
 			queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
 
 		cond_resched();
-- 
2.34.3