[PATCH v6 1/2] mm/vmstat: Use per cpu variable to track a vmstat discrepancy

[Aaron Tomlin <atomlin@redhat.com>]

This patch incorporates an idea from Marcelo's patch [1] where a
CPU-specific variable namely vmstat_dirty is used to indicate if
a vmstat imbalance is present for a given CPU. Therefore, at the
appropriate time, we can fold all the remaining differentials.

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

Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
---
 mm/vmstat.c | 46 +++++++++++++++-------------------------------
 1 file changed, 15 insertions(+), 31 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 373d2730fcf2..51564b7c85fe 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,
@@ -1873,6 +1883,7 @@ 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,
@@ -1937,6 +1948,7 @@ 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
@@ -1948,35 +1960,6 @@ 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
@@ -1990,7 +1973,7 @@ 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;
 
 	/*
@@ -2000,6 +1983,7 @@ void quiet_vmstat(void)
 	 * vmstat_shepherd will take care about that for us.
 	 */
 	refresh_cpu_vm_stats(false);
+	__this_cpu_write(vmstat_dirty, false);
 }
 
 /*
@@ -2021,7 +2005,7 @@ 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