[RFC PATCH 1/7] memcg: memcg_rstat_updated re-entrant safe against irqs

[Shakeel Butt <shakeel.butt@linux.dev>]

The function memcg_rstat_updated() is used to track the memcg stats
updates for optimizing the flushes. At the moment, it is not re-entrant
safe and the callers disabled irqs before calling. However to achieve
the goal of updating memcg stats without irqs, memcg_rstat_updated()
needs to be re-entrant safe against irqs.

This patch makes memcg_rstat_updated() re-entrant safe against irqs.
However it is using atomic_* ops which on x86, adds lock prefix to the
instructions. Since this is per-cpu data, the this_cpu_* ops are
preferred. However the percpu pointer is stored in struct mem_cgroup and
doing the upward traversal through struct mem_cgroup may cause two cache
misses as compared to traversing through struct memcg_vmstats_percpu
pointer.

NOTE: explore if there is atomic_* ops alternative without lock prefix.
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/memcontrol.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6cfa3550f300..2c4c095bf26c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -503,7 +503,7 @@ static inline int memcg_events_index(enum vm_event_item idx)
 
 struct memcg_vmstats_percpu {
 	/* Stats updates since the last flush */
-	unsigned int			stats_updates;
+	atomic_t			stats_updates;
 
 	/* Cached pointers for fast iteration in memcg_rstat_updated() */
 	struct memcg_vmstats_percpu	*parent;
@@ -590,12 +590,15 @@ static bool memcg_vmstats_needs_flush(struct memcg_vmstats *vmstats)
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 {
 	struct memcg_vmstats_percpu *statc;
-	int cpu = smp_processor_id();
-	unsigned int stats_updates;
+	int cpu;
+	int stats_updates;
 
 	if (!val)
 		return;
 
+	/* Don't assume callers have preemption disabled. */
+	cpu = get_cpu();
+
 	cgroup_rstat_updated(memcg->css.cgroup, cpu);
 	statc = this_cpu_ptr(memcg->vmstats_percpu);
 	for (; statc; statc = statc->parent) {
@@ -607,14 +610,16 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 		if (memcg_vmstats_needs_flush(statc->vmstats))
 			break;
 
-		stats_updates = READ_ONCE(statc->stats_updates) + abs(val);
-		WRITE_ONCE(statc->stats_updates, stats_updates);
+		stats_updates = atomic_add_return(abs(val), &statc->stats_updates);
 		if (stats_updates < MEMCG_CHARGE_BATCH)
 			continue;
 
-		atomic64_add(stats_updates, &statc->vmstats->stats_updates);
-		WRITE_ONCE(statc->stats_updates, 0);
+		stats_updates = atomic_xchg(&statc->stats_updates, 0);
+		if (stats_updates)
+			atomic64_add(stats_updates,
+				     &statc->vmstats->stats_updates);
 	}
+	put_cpu();
 }
 
 static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force)
@@ -4155,7 +4160,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
 		mem_cgroup_stat_aggregate(&ac);
 
 	}
-	WRITE_ONCE(statc->stats_updates, 0);
+	atomic_set(&statc->stats_updates, 0);
 	/* We are in a per-cpu loop here, only do the atomic write once */
 	if (atomic64_read(&memcg->vmstats->stats_updates))
 		atomic64_set(&memcg->vmstats->stats_updates, 0);
-- 
2.47.1