Re: [PATCH v2] mm: mlock: remove lru_add_drain_all()

[Balbir Singh]

On Fri, Oct 20, 2017 at 9:25 AM, Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> lru_add_drain_all() is not required by mlock() and it will drain
> everything that has been cached at the time mlock is called. And
> that is not really related to the memory which will be faulted in
> (and cached) and mlocked by the syscall itself.
>
> Without lru_add_drain_all() the mlocked pages can remain on pagevecs
> and be moved to evictable LRUs. However they will eventually be moved
> back to unevictable LRU by reclaim. So, we can safely remove
> lru_add_drain_all() from mlock syscall. Also there is no need for
> local lru_add_drain() as it will be called deep inside __mm_populate()
> (in follow_page_pte()).
>
> On larger machines the overhead of lru_add_drain_all() in mlock() can
> be significant when mlocking data already in memory. We have observed
> high latency in mlock() due to lru_add_drain_all() when the users
> were mlocking in memory tmpfs files.
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> ---

I'm afraid I still don't fully understand the impact in terms of numbers and
statistics as seen from inside a cgroup. My understanding is that we'll slowly
see the unreclaimable stats go up as we drain the pvec's across CPU's
I understand the optimization and I can see why lru_add_drain_all() is
expensive.

Acked-by: Balbir Singh <bsingharora@xxxxxxxxx>

Balbir Singh.