linux-kernel - Re: [PATCH v2] mm/swap: piggyback lru_add_drain

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Date:   Fri, 4 Oct 2019 17:06:13 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2] mm/swap: piggyback lru_add_drain_all() calls

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.

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.