The patch titled
Subject: mm/swap.c: piggyback lru_add_drain_all() calls
has been added to the -mm tree. Its filename is
mm-swap-piggyback-lru_add_drain_all-calls.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-swap-piggyback-lru_add_drain_all-calls.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-swap-piggyback-lru_add_drain_all-calls.patch
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From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Subject: mm/swap.c: piggyback lru_add_drain_all() calls
This is a very slow operation. Right now POSIX_FADV_DONTNEED is the top
user because it has to freeze page references when removing it from the
cache. invalidate_bdev() calls it for the same reason. Both are
triggered from userspace, so it's easy to generate a storm.
mlock/mlockall no longer calls lru_add_drain_all - I've seen here
serious slowdown on older kernels.
There are some less obvious paths in memory migration/CMA/offlining which
shouldn't call frequently.
The worst case requires a non-trivial workload because lru_add_drain_all()
skips cpus where vectors are empty. Something must constantly generate a
flow of pages for each cpu. Also cpus must be busy to make scheduling
per-cpu works slower. And the machine must be big enough (64+ cpus in our
case).
In our case that was a massive series of mlock calls in map-reduce while
other tasks write logs (and generates flows of new pages in per-cpu
vectors). Mlock calls were serialized by mutex and accumulated latency up
to 10 seconds or more.
The kernel does not call lru_add_drain_all on mlock paths since 4.15, but
the same scenario could be triggered by fadvise(POSIX_FADV_DONTNEED) or
any other remaining user.
There is no reason to do the drain again if somebody else already drained
all the per-cpu vectors while we waited for the lock.
Piggyback on a drain starting and finishing while we wait for the lock:
all pages pending at the time of our entry were drained from the vectors.
Callers like POSIX_FADV_DONTNEED retry their operations once after
draining per-cpu vectors when pages have unexpected references.
Link: http://lkml.kernel.org/r/157019456205.3142.3369423180908482020.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/swap.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
--- a/mm/swap.c~mm-swap-piggyback-lru_add_drain_all-calls
+++ a/mm/swap.c
@@ -708,9 +708,10 @@ static void lru_add_drain_per_cpu(struct
*/
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
_
Patches currently in -mm which might be from khlebnikov@xxxxxxxxxxxxxx are
mm-swap-piggyback-lru_add_drain_all-calls.patch
