linux-kernel - Re: [PATCH] mm: Make count list_lru_one::nr

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Date:   Thu, 28 Sep 2017 14:02:30 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     vdavydov.dev@...il.com, apolyakov@...et.ru,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aryabinin@...tuozzo.com
Subject: Re: [PATCH] mm: Make count list_lru_one::nr_items lockless

On Thu, 28 Sep 2017 10:48:55 +0300 Kirill Tkhai <ktkhai@...tuozzo.com> wrote:

> >> This patch aims to make super_cache_count() (and other functions,
> >> which count LRU nr_items) more effective.
> >> It allows list_lru_node::memcg_lrus to be RCU-accessed, and makes
> >> __list_lru_count_one() count nr_items lockless to minimize
> >> overhead introduced by locking operation, and to make parallel
> >> reclaims more scalable.
> > 
> > And...  what were the effects of the patch?  Did you not run the same
> > performance tests after applying it?
> 
> I've just detected the such high usage of shrink slab on production node. It's rather
> difficult to make it use another kernel, than it uses, only kpatches are possible.
> So, I haven't estimated how it acts on node's performance.
> On test node I see, that the patch obviously removes raw_spin_lock from perf profile.
> So, it's a little bit untested in this way.

Well that's a problem.  The patch increases list_lru.o text size by a
lot (4800->5696) which will have a cost.  And we don't have proof that
any benefit is worth that cost.  It shouldn't be too hard to cook up a
synthetic test to trigger memcg slab reclaim and then run a
before-n-after benchmark?