lkml.org 
[lkml]   [2012]   [Mar]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v3 2/2] memcg: avoid THP split in task migration
    On Thu, 8 Mar 2012 18:33:14 -0800 (PST)
    Hugh Dickins <hughd@google.com> wrote:

    > On Fri, 9 Mar 2012, KAMEZAWA Hiroyuki wrote:
    > > > +
    > > > + page = pmd_page(pmd);
    > > > + VM_BUG_ON(!page || !PageHead(page));
    > > > + if (!move_anon() || page_mapcount(page) != 1)
    > > > + return 0;
    > >
    > > Could you add this ?
    > > ==
    > > static bool move_check_shared_map(struct page *page)
    > > {
    > > /*
    > > * Handling of shared pages between processes is a big trouble in memcg.
    > > * Now, we never move shared-mapped pages between memcg at 'task' moving because
    > > * we have no hint which task the page is really belongs to. For example,
    > > * When a task does "fork()-> move to the child other group -> exec()", the charges
    > > * should be stay in the original cgroup.
    > > * So, check mapcount to determine we can move or not.
    > > */
    > > return page_mapcount(page) != 1;
    > > }
    >
    > That's a helpful elucidation, thank you. However...
    >
    > That is not how it has actually been behaving for the last 18 months
    > (because of the "> 2" bug), so in practice you are asking for a change
    > in behaviour there.
    >
    Yes.


    > And it's not how it has been and continues to behave with file pages.
    >
    It's ok to add somethink like..

    if (PageAnon(page) && !move_anon())
    return false;
    ...

    > Isn't getting that behaviour in fork-move-exec just a good reason not
    > to set move_charge_at_immigrate?
    >
    Hmm. Maybe.

    > I think there are other scenarios where you do want all the pages to
    > move if move_charge_at_immigrate: and that's certainly easier to
    > describe and to understand and to code.
    >
    > But if you do insist on not moving the shared, then it needs to involve
    > something like mem_cgroup_count_swap_user() on PageSwapCache pages,
    > rather than just the bare page_mapcount().
    >

    This 'moving swap account' was a requirement from a user (NEC?).
    But no user doesn't say 'I want to move shared pages between cgroups at task
    move !' and I don't like to move shared objects.

    > I'd rather delete than add code here!
    >

    As a user, for Fujitsu, I believe it's insane to move task between cgroups.
    So, I have no benefit from this code, at all.
    Ok, maybe I'm not a stakeholder,here.

    If users say all shared pages should be moved, ok, let's move.
    But change of behavior should be documented and implemented in an independet
    patch. CC'ed Nishimura-san, he implemetned this, a real user.

    Thanks,
    -Kame





    \
     
     \ /
      Last update: 2012-03-09 04:29    [W:0.055 / U:92.944 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site