[PATCH v8 2/5] mm/vmstat: Use vmstat_dirty to track CPU-specific vmstat discrepancies

[Aaron Tomlin <atomlin@redhat.com>]

This patch will now use the previously introduced CPU-specific variable
namely vmstat_dirty 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(). The idea is based on Marcelo's patch [1].

[1]: https://lore.kernel.org/lkml/20220204173554.763888172@fedora.localdomain/

Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
---
 mm/vmstat.c | 48 ++++++++++++++----------------------------------
 1 file changed, 14 insertions(+), 34 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 24c67b2e58fd..472175642bd9 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -383,6 +383,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 		x = 0;
 	}
 	__this_cpu_write(*p, x);
+	vmstat_mark_dirty();
 
 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
 		preempt_enable();
@@ -421,6 +422,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
 		x = 0;
 	}
 	__this_cpu_write(*p, x);
+	vmstat_mark_dirty();
 
 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
 		preempt_enable();
@@ -619,6 +621,7 @@ static inline void mod_zone_state(struct zone *zone,
 
 	if (z)
 		zone_page_state_add(z, zone, item);
+	vmstat_mark_dirty();
 }
 
 void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
@@ -687,6 +690,7 @@ static inline void mod_node_state(struct pglist_data *pgdat,
 
 	if (z)
 		node_page_state_add(z, pgdat, item);
+	vmstat_mark_dirty();
 }
 
 void mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
@@ -841,6 +845,14 @@ static int refresh_cpu_vm_stats(bool do_pagesets)
 	int global_node_diff[NR_VM_NODE_STAT_ITEMS] = { 0, };
 	int changes = 0;
 
+	/*
+	 * Clear vmstat_dirty before clearing the percpu vmstats.
+	 * If interrupts are enabled, it is possible that an interrupt
+	 * or another task modifies a percpu vmstat, which will
+	 * set vmstat_dirty to true.
+	 */
+	vmstat_clear_dirty();
+
 	for_each_populated_zone(zone) {
 		struct per_cpu_zonestat __percpu *pzstats = zone->per_cpu_zonestats;
 #ifdef CONFIG_NUMA
@@ -1971,35 +1983,6 @@ 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
@@ -2010,10 +1993,7 @@ void quiet_vmstat(void)
 	if (system_state != SYSTEM_RUNNING)
 		return;
 
-	if (!delayed_work_pending(this_cpu_ptr(&vmstat_work)))
-		return;
-
-	if (!need_update(smp_processor_id()))
+	if (!is_vmstat_dirty())
 		return;
 
 	/*
@@ -2044,7 +2024,7 @@ 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();
-- 
2.37.1