linux-kernel - Re: [PATCH] vmalloc: use rcu list iterator to reduce vmap_area

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Message-Id: <20140529130544.56213f048f331723329ff828@linux-foundation.org>
Date:	Thu, 29 May 2014 13:05:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andi Kleen <andi@...stfloor.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Richard Yao <ryao@...too.org>
Subject: Re: [PATCH] vmalloc: use rcu list iterator to reduce vmap_area_lock
 contention

On Thu, 29 May 2014 15:22:34 +0900 Joonsoo Kim <iamjoonsoo.kim@....com> wrote:

> Richard Yao reported a month ago that his system have a trouble
> with vmap_area_lock contention during performance analysis
> by /proc/meminfo. Andrew asked why his analysis checks /proc/meminfo
> stressfully, but he didn't answer it.
> 
> https://lkml.org/lkml/2014/4/10/416
> 
> Although I'm not sure that this is right usage or not, there is a solution
> reducing vmap_area_lock contention with no side-effect. That is just
> to use rcu list iterator in get_vmalloc_info(). This function only needs
> values on vmap_area structure, so we don't need to grab a spinlock.

The mixture of rcu protection and spinlock protection for
vmap_area_list is pretty confusing.  Are you able to describe the
overall design here?  When and why do we use one versus the other?


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