Messages in this thread | | | Date | Wed, 23 Jul 2014 07:44:49 -0400 | From | Johannes Weiner <> | Subject | Re: [PATCH 1/2] mm: Avoid full RCU lookup of memcg for statistics updates |
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On Wed, Jul 23, 2014 at 12:24:15PM +0100, Mel Gorman wrote: > When updating memcg VM statistics like PGFAULT we take the rcu read > lock and lookup the memcg. For statistic updates this is overkill > when the process may not belong to a memcg. This patch adds a light > check to check if a memcg potentially exists. It's race-prone in that > some VM stats may be missed when a process first joins a memcg but > that is not serious enough to justify a constant performance penalty.
Tasks always belong to a memcg, the root group per default. There isn't really any accounting that could be omitted.
> The exact impact of this is difficult to quantify because it's timing > sensitive, workload sensitive and sensitive to the RCU options set. However, > broadly speaking there should be less interference due to page fault > activity in both the number of RCU grace periods and their age. > > Signed-off-by: Mel Gorman <mgorman@suse.de> > --- > include/linux/memcontrol.h | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index eb65d29..76fa97d 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -220,6 +220,14 @@ static inline void mem_cgroup_count_vm_event(struct mm_struct *mm, > { > if (mem_cgroup_disabled()) > return; > + /* > + * For statistic updates it's overkill to take the RCU lock and do > + * a fully safe lookup of an associated memcg. Do a simple check > + * first. At worst, we miss a few stat updates when a process is > + * moved to a memcg for the first time. > + */ > + if (!rcu_access_pointer(mm->owner)) > + return;
mm->owner is set when the mm is first initialized, it's only NULL during race conditions on exit.
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