[PATCH 5/6] mm/page_alloc: Allow remote per-cpu page list draining

[Nicolas Saenz Julienne <nsaenzju@redhat.com>]

Some setups, notably NOHZ_FULL CPUs, are too busy to handle the per-cpu
drain work queued by __drain_all_pages(). So introduce new a mechanism
to remotely drain the per-cpu lists. It is made possible by using a more
constraining locking scheme, which is disabled by default and can be
enabled through the 'remote_pcpu_cache_access' static key. Regular users
shouldn't see any functional or performance change. Upcoming patches
will make use of the static key.

This is based on previous work by Thomas Gleixner, Anna-Maria Gleixner,
and Sebastian Andrzej Siewior[1].

[1] https://patchwork.kernel.org/project/linux-mm/patch/20190424111208.24459-3-bigeasy@linutronix.de/
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
---
 mm/page_alloc.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3244eb2ab51b..717df675ea06 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3268,15 +3268,25 @@ static void __drain_all_pages(struct zone *zone, bool force_all_cpus)
 			cpumask_clear_cpu(cpu, &cpus_with_pcps);
 	}
 
-	for_each_cpu(cpu, &cpus_with_pcps) {
-		struct pcpu_drain *drain = per_cpu_ptr(&pcpu_drain, cpu);
+	if (static_branch_unlikely(&remote_pcpu_cache_access)) {
+		for_each_cpu(cpu, &cpus_with_pcps) {
+			if (zone) {
+				drain_pages_zone(cpu, zone);
+			} else {
+				drain_pages(cpu);
+			}
+		}
+	} else {
+		for_each_cpu(cpu, &cpus_with_pcps) {
+			struct pcpu_drain *drain = per_cpu_ptr(&pcpu_drain, cpu);
 
-		drain->zone = zone;
-		INIT_WORK(&drain->work, drain_local_pages_wq);
-		queue_work_on(cpu, mm_percpu_wq, &drain->work);
+			drain->zone = zone;
+			INIT_WORK(&drain->work, drain_local_pages_wq);
+			queue_work_on(cpu, mm_percpu_wq, &drain->work);
+		}
+		for_each_cpu(cpu, &cpus_with_pcps)
+			flush_work(&per_cpu_ptr(&pcpu_drain, cpu)->work);
 	}
-	for_each_cpu(cpu, &cpus_with_pcps)
-		flush_work(&per_cpu_ptr(&pcpu_drain, cpu)->work);
 
 	mutex_unlock(&pcpu_drain_mutex);
 }
-- 
2.31.1