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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>
This commit is contained in:
Konstantin Khlebnikov 2019-11-30 17:50:40 -08:00 committed by Linus Torvalds
parent 408a60eddd
commit eef1a429f2
1 changed files with 15 additions and 1 deletions
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16
mm/swap.c
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@ -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