Re: [PATCH v2] mm/swap: piggyback lru_add_drain_all() calls

[Michal Hocko]

On Fri 04-10-19 17:06:13, Konstantin Khlebnikov wrote:
> On 04/10/2019 16.39, Michal Hocko wrote:
> > On Fri 04-10-19 16:32:39, Konstantin Khlebnikov wrote:
> > > On 04/10/2019 16.12, Michal Hocko wrote:
> > > > On Fri 04-10-19 16:09:22, Konstantin Khlebnikov wrote:
> > > > > This is very slow operation. There is no reason to do it again if somebody
> > > > > else already drained all per-cpu vectors while we waited for 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.
> > > > > 
> > > > > Callers like POSIX_FADV_DONTNEED retry their operations once after
> > > > > draining per-cpu vectors when pages have unexpected references.
> > > > 
> > > > This describes why we need to wait for preexisted pages on the pvecs but
> > > > the changelog doesn't say anything about improvements this leads to.
> > > > In other words what kind of workloads benefit from it?
> > > 
> > > Right now POSIX_FADV_DONTNEED is top user because it have to freeze page
> > > reference when removes it from cache. invalidate_bdev calls it for same reason.
> > > Both are triggered from userspace, so it's easy to generate storm.
> > > 
> > > mlock/mlockall no longer calls lru_add_drain_all - I've seen here
> > > serious slowdown on older kernel.
> > > 
> > > There are some less obvious paths in memory migration/CMA/offlining
> > > which shouldn't be called frequently.
> > 
> > Can you back those claims by any numbers?
> > 
> 
> Well, worst case requires non-trivial workload because lru_add_drain_all
> skips cpus where vectors are empty. Something must constantly generates
> flow of pages at each cpu. Also cpus must be busy to make scheduling per-cpu
> works slower. And machine must be big enough (64+ cpus in our case).
> 
> In our case that was massive series of mlock calls in map-reduce while other
> tasks writes log (and generates flow of new pages in per-cpu vectors). Mlock
> calls were serialized by mutex and accumulated latency up to 10 second and more.

This is a very useful information!

> Kernel does not call lru_add_drain_all on mlock paths since 4.15, but same scenario
> could be triggered by fadvise(POSIX_FADV_DONTNEED) or any other remaining user.

OK, so I read it as, you are unlikely to hit problems with the current
tree but they are still possible in principle. That is a useful
information as well. All that belongs to the changelog. Do not let us
guess and future generations scratch their heads WTH is going on with
that weird code.

Thanks!
-- 
Michal Hocko
SUSE Labs