linux-kernel - Re: [PATCH rebase] mm: fix vm-scalability regression in cgroup-aware workingset code

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Message-ID: <20160627130527.GK31799@dhcp22.suse.cz>
Date:	Mon, 27 Jun 2016 15:05:28 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ye Xiaolong <xiaolong.ye@...el.com>,
	Vladimir Davydov <vdavydov@...tuozzo.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH rebase] mm: fix vm-scalability regression in cgroup-aware
 workingset code

[Sorry for a late reply]

On Fri 24-06-16 13:51:01, Johannes Weiner wrote:
> This is a rebased version on top of mmots sans the nodelru stuff.
> 
> ---
> 
> 23047a96d7cf ("mm: workingset: per-cgroup cache thrash detection")
> added a page->mem_cgroup lookup to the cache eviction, refault, and
> activation paths, as well as locking to the activation path, and the
> vm-scalability tests showed a regression of -23%. While the test in
> question is an artificial worst-case scenario that doesn't occur in
> real workloads - reading two sparse files in parallel at full CPU
> speed just to hammer the LRU paths - there is still some optimizations
> that can be done in those paths.
> 
> Inline the lookup functions to eliminate calls. Also, page->mem_cgroup
> doesn't need to be stabilized when counting an activation; we merely
> need to hold the RCU lock to prevent the memcg from being freed.
> 
> This cuts down on overhead quite a bit:
> 
> 23047a96d7cfcfca 063f6715e77a7be5770d6081fe
> ---------------- --------------------------
>          %stddev     %change         %stddev
>              \          |                \
>   21621405 +- 0%     +11.3%   24069657 +- 2%  vm-scalability.throughput
> 
> Reported-by: Ye Xiaolong <xiaolong.ye@...el.com>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Michal Hocko <mhocko@...e.com>

Minor note below

> +static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
> +{

I guess rcu_read_lock_held() here would be appropriate

> +	return READ_ONCE(page->mem_cgroup);
> +}
-- 
Michal Hocko
SUSE Labs