Re: [PATCH 6/7] cgroups: Add res counter common ancestor searching

[Frederic Weisbecker]

On Mon, Jul 25, 2011 at 06:05:00PM -0700, Paul Menage wrote:
> On Mon, Jul 11, 2011 at 7:15 AM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> > Add a new API to find the common ancestor between two resource
> > counters. This includes the passed resource counter themselves.
> >
> > Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> 
> Acked-by: Paul Menage <menage@xxxxxxxxxx>
> 
> There's something distasteful about doing an O(N^2) search on every
> charge/uncharge operation, but I guess in general N isn't going to be
> more than 2 or 3 so it's not worth optimizing.
> 
> If you wanted to avoid the N^2 search then you could add a "depth"
> field in struct res_counter, which would be initialized to
> parent->depth+1, and then easily get both r1 and r2 to the same depth
> before proceeding up the tree in lock step, but that would bloat all
> instances of res_counter, which might work out as more expensive in
> the long run.

Yeah it only happens on slow paths and probably cgroups hierarchies don't
tend to be so deep.

But if that's a problem we can still optimize the way you said, and perhaps
move that depth field into the struct task_counter instead.

Thanks.

> 
> Paul
> 
> > Cc: Paul Menage <menage@xxxxxxxxxx>
> > Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Aditya Kali <adityakali@xxxxxxxxxx>
> > ---
> >  include/linux/res_counter.h |    2 ++
> >  kernel/res_counter.c        |   19 +++++++++++++++++++
> >  2 files changed, 21 insertions(+), 0 deletions(-)
> >
> > diff --git a/include/linux/res_counter.h b/include/linux/res_counter.h
> > index 8c421ac..354ed30 100644
> > --- a/include/linux/res_counter.h
> > +++ b/include/linux/res_counter.h
> > @@ -139,6 +139,8 @@ void res_counter_uncharge_until(struct res_counter *counter, struct res_counter
> >                                unsigned long val);
> >  void res_counter_uncharge(struct res_counter *counter, unsigned long val);
> >
> > +struct res_counter *res_counter_common_ancestor(struct res_counter *l, struct res_counter *r);
> > +
> >  /**
> >  * res_counter_margin - calculate chargeable space of a counter
> >  * @cnt: the counter
> > diff --git a/kernel/res_counter.c b/kernel/res_counter.c
> > index 39f2513..1b2efd6 100644
> > --- a/kernel/res_counter.c
> > +++ b/kernel/res_counter.c
> > @@ -102,6 +102,25 @@ void res_counter_uncharge(struct res_counter *counter, unsigned long val)
> >        res_counter_uncharge_until(counter, NULL, val);
> >  }
> >
> > +struct res_counter *
> > +res_counter_common_ancestor(struct res_counter *r1, struct res_counter *r2)
> > +{
> > +       struct res_counter *iter;
> > +
> > +       while (r1) {
> > +               iter = r2;
> > +               while (iter) {
> > +                       if (iter == r1)
> > +                               return iter;
> > +                       iter = iter->parent;
> > +               }
> > +
> > +               r1 = r1->parent;
> > +       }
> > +
> > +       return NULL;
> > +}
> > +
> >  static inline unsigned long long *
> >  res_counter_member(struct res_counter *counter, int member)
> >  {
> > --
> > 1.7.5.4
> >
> >
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