linux-kernel - Re: [PATCH v2] vmalloc: use rcu list iterator to reduce vmap_area

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Date:	Wed, 11 Jun 2014 14:56:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	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>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2] vmalloc: use rcu list iterator to reduce
 vmap_area_lock contention

On Wed, 11 Jun 2014 13:34:04 +0900 Joonsoo Kim <iamjoonsoo.kim@....com> wrote:

> > While rcu list traversal over the vmap_area_list is safe, this may
> > arrive at different results than the spinlocked version. The rcu list
> > traversal version will not be a 'snapshot' of a single, valid instant
> > of the entire vmap_area_list, but rather a potential amalgam of
> > different list states.
> 
> Hello,
> 
> Yes, you are right, but I don't think that we should be strict here.
> Meminfo is already not a 'snapshot' at specific time. While we try to
> get certain stats, the other stats can change.
> And, although we may arrive at different results than the spinlocked
> version, the difference would not be large and would not make serious
> side-effect.

mm, well...  The spinlocked version will at least report a number which
*used* to be true.  The new improved racy version could for example see
a bunch of new allocations but fail to see the bunch of frees which
preceded those new allocations.  Net result: it reports allocation
totals which exceed anything which this kernel has ever sustained.

But hey, it's only /proc/meminfo:VmallocFoo.  I'll eat my hat if anyone
cares about it.
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