On 01/06/2020 17.37, Hillf Danton wrote:
After updating the lru drain sequence, new comers avoid waiting for
the current drainer, because he is flushing works on each online CPU,
by trying to lock the mutex; the drainer OTOH tries to do works for
those who fail to acquire the lock by checking the lru drain sequence
after releasing lock.
See eef1a429f234 ("mm/swap.c: piggyback lru_add_drain_all() calls")
for reasons why we can skip waiting for the lock.
That patch tells nothing about such change in behaviour.
Callers like invalidate_bdev() really need synchronous drain to be sure
that pages have no extra reference from per-cpu vectors.
The memory barriers around the sequence and the lock come together
to remove waiters without their drain works bandoned.
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Signed-off-by: Hillf Danton <hdanton@xxxxxxxx>
---
This is inspired by one of the works from Sebastian.
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -714,10 +714,11 @@ static void lru_add_drain_per_cpu(struct
*/
void lru_add_drain_all(void)
{
- static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
+ static unsigned int lru_drain_seq;
static DEFINE_MUTEX(lock);
static struct cpumask has_work;
- int cpu, seq;
+ int cpu;
+ unsigned int seq;
/*
* Make sure nobody triggers this path before mm_percpu_wq is fully
@@ -726,18 +727,16 @@ void lru_add_drain_all(void)
if (WARN_ON(!mm_percpu_wq))
return;
- seq = raw_read_seqcount_latch(&seqcount);
+ lru_drain_seq++;
+ smp_mb();
- mutex_lock(&lock);
+more_work:
- /*
- * 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;
+ if (!mutex_trylock(&lock))
+ return;
- raw_write_seqcount_latch(&seqcount);
+ smp_mb();
+ seq = lru_drain_seq;
cpumask_clear(&has_work);
@@ -759,8 +758,11 @@ 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);
+
+ smp_mb();
+ if (seq != lru_drain_seq)
+ goto more_work;
}
#else
void lru_add_drain_all(void)
--
