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Subject[PATCH v2] mm/swap: piggyback lru_add_drain_all() calls
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This is very slow operation. There is no reason to do it again if somebody
else already drained all per-cpu vectors while we waited for lock.

Piggyback on drain started and finished while we waited for lock:
all pages pended at the time of our enter were drained from vectors.

Callers like POSIX_FADV_DONTNEED retry their operations once after
draining per-cpu vectors when pages have unexpected references.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
mm/swap.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/swap.c b/mm/swap.c
index 38c3fa4308e2..5ba948a9d82a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -708,9 +708,10 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
*/
void lru_add_drain_all(void)
{
+ static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
static DEFINE_MUTEX(lock);
static struct cpumask has_work;
- int cpu;
+ int cpu, seq;

/*
* Make sure nobody triggers this path before mm_percpu_wq is fully
@@ -719,7 +720,19 @@ void lru_add_drain_all(void)
if (WARN_ON(!mm_percpu_wq))
return;

+ seq = raw_read_seqcount_latch(&seqcount);
+
mutex_lock(&lock);
+
+ /*
+ * Piggyback on drain started and finished while we waited for lock:
+ * all pages pended at the time of our enter were drained from vectors.
+ */
+ if (__read_seqcount_retry(&seqcount, seq))
+ goto done;
+
+ raw_write_seqcount_latch(&seqcount);
+
cpumask_clear(&has_work);

for_each_online_cpu(cpu) {
@@ -740,6 +753,7 @@ void lru_add_drain_all(void)
for_each_cpu(cpu, &has_work)
flush_work(&per_cpu(lru_add_drain_work, cpu));

+done:
mutex_unlock(&lock);
}
#else
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