[PATCH 1/2] memcg: flush stats only if updated

[Shakeel Butt]

At the moment, the kernel flushes the memcg stats on every refault and
also on every reclaim iteration. Although rstat maintains per-cpu update
tree but on the flush the kernel still has to go through all the cpu
rstat update tree to check if there is anything to flush. This patch
adds the tracking on the stats update side to make flush side more
clever by skipping the flush if there is no update.

The stats update codepath is very sensitive performance wise for many
workloads and benchmarks. So, we can not follow what the commit
aa48e47e3906 ("memcg: infrastructure to flush memcg stats") did which
was triggering async flush through queue_work() and caused a lot
performance regression reports. That got reverted by the commit
1f828223b799 ("memcg: flush lruvec stats in the refault").

In this patch we kept the stats update codepath very minimal and let the
stats reader side to flush the stats only when the updates are over a
specific threshold. For now the threshold is (nr_cpus * CHARGE_BATCH).

To evaluate the impact of this patch, an 8 GiB tmpfs file is created on
a system with swap-on-zram and the file was pushed to swap through
memory.force_empty interface. On reading the whole file, the memcg stat
flush in the refault code path is triggered. With this patch, we
bserved 63% reduction in the read time of 8 GiB file.

Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
 mm/memcontrol.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6da5020a8656..933dde29c67b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -107,6 +107,8 @@ static bool do_memsw_account(void)
 static void flush_memcg_stats_dwork(struct work_struct *w);
 static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork);
 static DEFINE_SPINLOCK(stats_flush_lock);
+static DEFINE_PER_CPU(unsigned int, stats_updates);
+static atomic_t stats_flush_threshold = ATOMIC_INIT(0);
 
 #define THRESHOLDS_EVENTS_TARGET 128
 #define SOFTLIMIT_EVENTS_TARGET 1024
@@ -635,6 +637,13 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
 	return mz;
 }
 
+static inline void memcg_rstat_updated(struct mem_cgroup *memcg)
+{
+	cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
+	if (!(__this_cpu_inc_return(stats_updates) % MEMCG_CHARGE_BATCH))
+		atomic_inc(&stats_flush_threshold);
+}
+
 /**
  * __mod_memcg_state - update cgroup memory statistics
  * @memcg: the memory cgroup
@@ -647,7 +656,7 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val)
 		return;
 
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
-	cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
+	memcg_rstat_updated(memcg);
 }
 
 /* idx can be of type enum memcg_stat_item or node_stat_item. */
@@ -675,10 +684,12 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	memcg = pn->memcg;
 
 	/* Update memcg */
-	__mod_memcg_state(memcg, idx, val);
+	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
 
 	/* Update lruvec */
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
+
+	memcg_rstat_updated(memcg);
 }
 
 /**
@@ -780,7 +791,7 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 		return;
 
 	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
-	cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
+	memcg_rstat_updated(memcg);
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -5341,15 +5352,22 @@ static void mem_cgroup_css_reset(struct cgroup_subsys_state *css)
 	memcg_wb_domain_size_changed(memcg);
 }
 
-void mem_cgroup_flush_stats(void)
+static void __mem_cgroup_flush_stats(void)
 {
 	if (!spin_trylock(&stats_flush_lock))
 		return;
 
 	cgroup_rstat_flush_irqsafe(root_mem_cgroup->css.cgroup);
+	atomic_set(&stats_flush_threshold, 0);
 	spin_unlock(&stats_flush_lock);
 }
 
+void mem_cgroup_flush_stats(void)
+{
+	if (atomic_read(&stats_flush_threshold) > num_online_cpus())
+		__mem_cgroup_flush_stats();
+}
+
 static void flush_memcg_stats_dwork(struct work_struct *w)
 {
 	mem_cgroup_flush_stats();
-- 
2.33.0.685.g46640cef36-goog