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

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Message-ID: <20131028113120.GB11541@mtj.dyndns.org>
Date:	Mon, 28 Oct 2013 07:31:20 -0400
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 Mon, Oct 28, 2013 at 11:00:55AM +0800, Wei Yang wrote:
> >Does this actually matter?  If so, it'd probably make a lot more sense
> >to start inner loop at @cpu + 1 so that it becomes O(N).
> 
> One of the worst case in my mind:
> 
> CPU:        0    1    2    3    4    ...
> Group:      0    1    2    3    4    ...
> (sounds it is impossible in the real world)

I was wondering whether you had an actual case where this actually
matters or it's just something you thought of while reading the code.

> Every time, when we encounter a new CPU and try to assign it to a group, we
> found it belongs to a new group. The original logic will iterate on all old
> CPUs again, while the new logic could skip this and assign it to a new group.
> 
> Again, this is a tiny change, which doesn't matters a lot.

I think it *could* matter because the current implementation is O(N^2)
where N is the number of CPUs.  On machines, say, with 4k CPU, it's
gonna loop 16M times but then again even that takes only a few
millisecs on modern machines.

> BTW, I don't get your point for "start inner loop at @cpu+1".
> 
> The original logic is:
> 	loop 1:   0 - nr_cpus
> 	loop 2:      0 - (cpu - 1)
> 
> If you found one better approach to improve the logic, I believe all the users
> will appreciate your efforts :-)

Ooh, right, I forgot about the break and then I thought somehow that
would make it O(N).  Sorry about that.  I blame jetlag. :)

Yeah, I don't know.  The function is quite hairy which makes me keep
things simpler and reluctant to make changes unless it actually makes
non-trivial difference.  The change looks okay to me but it seems
neither necessary or substantially beneficial and if my experience is
anything to go by, *any* change involves some risk of brekage no
matter how innocent it may look, so given the circumstances, I'd like
to keep things the way they are.

Thanks a lot!

-- 
tejun
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