Re: [PATCH 1/3] percpu: stop the loop when a cpu belongs to a newgroup

[Wei Yang]

On Mon, Oct 28, 2013 at 07:31:20AM -0400, Tejun Heo wrote:
>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.

Tejun,

Thanks for your comments.

I found this just in code review. :-)

>
>> 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.

I am not familiar with the real cases of the CPU numbers. Thanks for leting me
know there could be 4K CPUs.

Yep, a few millisecs sounds not a big a mount.

>
>> 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.

Yep, I really agree with you. If no big improvement, it is really not
necessary to change the code, which will face some risk.

Here I have another one, which in my mind will improve it in one case. Looking
forward to your comments :-) If I am not correct, please let me know. :-)