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 @@ -713,9 +713,10 @@ 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 @@ -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 _
