linux-kernel - Re: [PATCH 1/3] percpu: stop the loop when a cpu belongs to a new group

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Date:	Wed, 20 Nov 2013 00:51:21 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Wei Yang <weiyang@...ux.vnet.ibm.com>
Cc:	cl@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] percpu: stop the loop when a cpu belongs to a new
 group

Hello,

On Wed, Nov 20, 2013 at 11:00:56AM +0800, Wei Yang wrote:
> What do you think about this one?
> 
> >
> >From bd70498b9df47b25ff20054e24bb510c5430c0c3 Mon Sep 17 00:00:00 2001
> >From: Wei Yang <weiyang@...ux.vnet.ibm.com>
> >Date: Thu, 10 Oct 2013 09:42:14 +0800
> >Subject: [PATCH] percpu: optimize group assignment when cpu_distance_fn is
> > NULL
> >
> >When cpu_distance_fn is NULL, all CPUs belongs to group 0. The original logic
> >will continue to go through each CPU and its predecessor. cpu_distance_fn is
> >always NULL when pcpu_build_alloc_info() is called from pcpu_page_first_chunk().
> >
> >By applying this patch, the time complexity will drop to O(n) form O(n^2) in
> >case cpu_distance_fn is NULL.

The test was put in the inner loop because the nesting was already too
deep and cpu_distance_fn is unlikely to be NULL on machines where the
number of CPUs is high enough to matter.  If that O(n^2) loop is gonna
be a problem, it's gonna be a problem on large NUMA machines and we'll
have to do something about it for cases where cpu_distance_fn exists
anyway.

The patch is just extremely marginal.  Ah well... why not?  I'll apply
it once -rc1 drops.

Thanks.

-- 
tejun
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