lkml.org 
[lkml]   [2011]   [Jul]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 3/4] memcg: get rid of percpu_charge_mutex lock
    On Thu 21-07-11 19:30:51, KAMEZAWA Hiroyuki wrote:
    > On Thu, 21 Jul 2011 09:58:24 +0200
    > Michal Hocko <mhocko@suse.cz> wrote:
    >
    > > percpu_charge_mutex protects from multiple simultaneous per-cpu charge
    > > caches draining because we might end up having too many work items.
    > > At least this was the case until 26fe6168 (memcg: fix percpu cached
    > > charge draining frequency) when we introduced a more targeted draining
    > > for async mode.
    > > Now that also sync draining is targeted we can safely remove mutex
    > > because we will not send more work than the current number of CPUs.
    > > FLUSHING_CACHED_CHARGE protects from sending the same work multiple
    > > times and stock->nr_pages == 0 protects from pointless sending a work
    > > if there is obviously nothing to be done. This is of course racy but we
    > > can live with it as the race window is really small (we would have to
    > > see FLUSHING_CACHED_CHARGE cleared while nr_pages would be still
    > > non-zero).
    > > The only remaining place where we can race is synchronous mode when we
    > > rely on FLUSHING_CACHED_CHARGE test which might have been set by other
    > > drainer on the same group but we should wait in that case as well.
    > >
    > > Signed-off-by: Michal Hocko <mhocko@suse.cz>
    >
    > A concern.
    >
    > > ---
    > > mm/memcontrol.c | 12 ++----------
    > > 1 files changed, 2 insertions(+), 10 deletions(-)
    > >
    > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > > index 8180cd9..9d49a12 100644
    > > --- a/mm/memcontrol.c
    > > +++ b/mm/memcontrol.c
    > > @@ -2065,7 +2065,6 @@ struct memcg_stock_pcp {
    > > #define FLUSHING_CACHED_CHARGE (0)
    > > };
    > > static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
    > > -static DEFINE_MUTEX(percpu_charge_mutex);
    > >
    > > /*
    > > * Try to consume stocked charge on this cpu. If success, one page is consumed
    > > @@ -2166,7 +2165,8 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
    > >
    > > for_each_online_cpu(cpu) {
    > > struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
    > > - if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
    > > + if (root_mem == stock->cached &&
    > > + test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
    > > flush_work(&stock->work);
    >
    > Doesn't this new check handle hierarchy ?
    > css_is_ancestor() will be required if you do this check.

    Yes you are right. Will fix it. I will add a helper for the check.

    > BTW, this change should be in other patch, I think.

    I have put the change here intentionally because previously we were
    protected by the lock so we couldn't race with somebody else so the
    check was not necessary.

    >
    > Thanks,
    > -Kame

    Thanks
    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


    \
     
     \ /
      Last update: 2011-07-21 13:49    [W:0.023 / U:29.484 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site