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authorKonstantin Khlebnikov <khlebnikov@yandex-team.ru>2019-11-30 17:50:40 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2019-12-01 06:29:19 -0800
commiteef1a429f234f8f50e89226705bb47d7d3f1b840 (patch)
tree29c0aab64d3e989f42c47fed4ea87875cd50726f /mm/swap.c
parent408a60eddd206134fd306dfbc53bbde093b8deb0 (diff)
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@yandex-team.ru> Reviewed-by: Andrew Morton <akpm@linux-foundation.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Matthew Wilcox <willy@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/swap.c')
-rw-r--r--mm/swap.c16
1 files changed, 15 insertions, 1 deletions
diff --git a/mm/swap.c b/mm/swap.c
index 66e3c5033688..5341ae93861f 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -713,9 +713,10 @@ 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
@@ -724,7 +725,19 @@ 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) {
@@ -745,6 +758,7 @@ 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