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

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Message-Id: <20170927141530.25286286fb92a2573c4b548f@linux-foundation.org>
Date:   Wed, 27 Sep 2017 14:15:30 -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 Tue, 19 Sep 2017 18:06:33 +0300 Kirill Tkhai <ktkhai@...tuozzo.com> wrote:

> During the reclaiming slab of a memcg, shrink_slab iterates
> over all registered shrinkers in the system, and tries to count
> and consume objects related to the cgroup. In case of memory
> pressure, this behaves bad: I observe high system time and
> time spent in list_lru_count_one() for many processes on RHEL7
> kernel (collected via $perf record --call-graph fp -j k -a):
> 
> 0,50%  nixstatsagent  [kernel.vmlinux]  [k] _raw_spin_lock                [k] _raw_spin_lock
> 0,26%  nixstatsagent  [kernel.vmlinux]  [k] shrink_slab                   [k] shrink_slab
> 0,23%  nixstatsagent  [kernel.vmlinux]  [k] super_cache_count             [k] super_cache_count
> 0,15%  nixstatsagent  [kernel.vmlinux]  [k] __list_lru_count_one.isra.2   [k] _raw_spin_lock
> 0,15%  nixstatsagent  [kernel.vmlinux]  [k] list_lru_count_one            [k] __list_lru_count_one.isra.2
> 
> 0,94%  mysqld         [kernel.vmlinux]  [k] _raw_spin_lock                [k] _raw_spin_lock
> 0,57%  mysqld         [kernel.vmlinux]  [k] shrink_slab                   [k] shrink_slab
> 0,51%  mysqld         [kernel.vmlinux]  [k] super_cache_count             [k] super_cache_count
> 0,32%  mysqld         [kernel.vmlinux]  [k] __list_lru_count_one.isra.2   [k] _raw_spin_lock
> 0,32%  mysqld         [kernel.vmlinux]  [k] list_lru_count_one            [k] __list_lru_count_one.isra.2
> 
> 0,73%  sshd           [kernel.vmlinux]  [k] _raw_spin_lock                [k] _raw_spin_lock
> 0,35%  sshd           [kernel.vmlinux]  [k] shrink_slab                   [k] shrink_slab
> 0,32%  sshd           [kernel.vmlinux]  [k] super_cache_count             [k] super_cache_count
> 0,21%  sshd           [kernel.vmlinux]  [k] __list_lru_count_one.isra.2   [k] _raw_spin_lock
> 0,21%  sshd           [kernel.vmlinux]  [k] list_lru_count_one            [k] __list_lru_count_one.isra.2
> 
> 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?