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| H A D | swap.c | diff eef1a429f234f8f50e89226705bb47d7d3f1b840 Sun Dec 01 02:50:40 CET 2019 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> 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>
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