[PATCH 3/6] mm/swap: Allow remote LRU cache 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 __lru_add_drain_all(). 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 key.

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

[1] https://patchwork.kernel.org/project/linux-mm/cover/20190424111208.24459-1-bigeasy@linutronix.de/
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
---
 mm/swap.c | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index bcf73bd563a6..59e96a2520d5 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -915,19 +915,29 @@ inline void __lru_add_drain_all(bool force_all_cpus)
 	WRITE_ONCE(lru_drain_gen, lru_drain_gen + 1);
 	smp_mb();
 
-	cpumask_clear(&has_work);
-	for_each_online_cpu(cpu) {
-		struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
-
-		if (force_all_cpus || lru_cpu_needs_drain(cpu)) {
-			INIT_WORK(work, lru_add_drain_per_cpu);
-			queue_work_on(cpu, mm_percpu_wq, work);
-			__cpumask_set_cpu(cpu, &has_work);
+	if (static_branch_unlikely(&remote_pcpu_cache_access)) {
+		for_each_online_cpu(cpu) {
+			if (force_all_cpus || lru_cpu_needs_drain(cpu)) {
+				lru_cache_lock_cpu(&lru_pvecs.locks, cpu);
+				lru_add_drain_cpu(cpu);
+				lru_cache_unlock_cpu(&lru_pvecs.locks, cpu);
+			}
+		}
+	} else {
+		cpumask_clear(&has_work);
+		for_each_online_cpu(cpu) {
+			struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
+
+			if (force_all_cpus || lru_cpu_needs_drain(cpu)) {
+				INIT_WORK(work, lru_add_drain_per_cpu);
+				queue_work_on(cpu, mm_percpu_wq, work);
+				__cpumask_set_cpu(cpu, &has_work);
+			}
 		}
-	}
 
-	for_each_cpu(cpu, &has_work)
-		flush_work(&per_cpu(lru_add_drain_work, cpu));
+		for_each_cpu(cpu, &has_work)
+			flush_work(&per_cpu(lru_add_drain_work, cpu));
+	}
 
 done:
 	mutex_unlock(&lock);
-- 
2.31.1