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

[Wei Yang <weiyang@linux.vnet.ibm.com>]

On Mon, Oct 28, 2013 at 11:17:46PM +0800, Wei Yang wrote:
>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. :-)

Tejun,

What do you think about this one?

>
>From bd70498b9df47b25ff20054e24bb510c5430c0c3 Mon Sep 17 00:00:00 2001
>From: Wei Yang <weiyang@linux.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.
>
>Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
>---
> mm/percpu.c |   23 ++++++++++++-----------
> 1 files changed, 12 insertions(+), 11 deletions(-)
>
>diff --git a/mm/percpu.c b/mm/percpu.c
>index f79c807..8e6034f 100644
>--- a/mm/percpu.c
>+++ b/mm/percpu.c
>@@ -1481,20 +1481,21 @@ static struct pcpu_alloc_info * __init pcpu_build_alloc_info(
> 	for_each_possible_cpu(cpu) {
> 		group = 0;
> 	next_group:
>-		for_each_possible_cpu(tcpu) {
>-			if (cpu == tcpu)
>-				break;
>-			if (group_map[tcpu] == group && cpu_distance_fn &&
>-			    (cpu_distance_fn(cpu, tcpu) > LOCAL_DISTANCE ||
>-			     cpu_distance_fn(tcpu, cpu) > LOCAL_DISTANCE)) {
>-				group++;
>-				if (group == nr_groups) {
>-					nr_groups++;
>+		if (cpu_distance_fn)
>+			for_each_possible_cpu(tcpu) {
>+				if (cpu == tcpu)
> 					break;
>+				if (group_map[tcpu] == group &&
>+				    (cpu_distance_fn(cpu, tcpu) > LOCAL_DISTANCE ||
>+				     cpu_distance_fn(tcpu, cpu) > LOCAL_DISTANCE)) {
>+					group++;
>+					if (group == nr_groups) {
>+						nr_groups++;
>+						break;
>+					}
>+					goto next_group;
> 				}
>-				goto next_group;
> 			}
>-		}
> 		group_map[cpu] = group;
> 		group_cnt[group]++;
> 	}
>-- 
>1.7.5.4
>
>BTW, this one is based on my previous patch.
>
>>
>>Thanks a lot!
>>
>>-- 
>>tejun
>
>-- 
>Richard Yang
>Help you, Help me

-- 
Richard Yang
Help you, Help me

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