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    SubjectRe: [PATCH 1/3] percpu: stop the loop when a cpu belongs to a new group
    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. :-)

    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



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