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 <[email protected]> > --- > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
