Re: [RFD 3/9] Display /proc/stat information per cgroup

[Glauber Costa]

On 09/27/2011 07:21 PM, Peter Zijlstra wrote:
> On Tue, 2011-09-27 at 15:42 -0300, Glauber Costa wrote:
> > > > +static inline void task_cgroup_account_field(struct task_struct *p,
> > > > +                                            cputime64_t tmp, int index)
> > > > +{
> > > > +       struct kernel_stat *kstat;
> > > > +       struct task_group *tg = task_group(p);
> > > > +
> > > > +       do {
> > > > +               kstat = this_cpu_ptr(tg->cpustat);
> > > > +               kstat->cpustat[index] = cputime64_add(kstat->cpustat[index],
> > > > +                                                     tmp);
> > > > +               tg = tg->parent;
> > > > +       } while (tg);
> > > > +}
> > >
> > > What protects the walk (tg = tg->parent)? Could you please document it
> > I think that the fact that the hierarchy only grows down, thus parent
> > never changes (or am I wrong?)
> >
> > And since we run all this with preempt disabled and with the runqueue
> > locked, we should have no problems.
> >
> > Do you agree?
>
> Right, so the tg can't be destroyed unless its empty, us finding this
> task in it means its not empty, we require rq->lock or p->pi_lock to
> move the task.
>
> However, afaict we don't actually have any of those locks.
>
> That said, it should be sufficient to wrap the whole thing in
> rcu_read_lock(), accounting one tick funny because a cgroup move race
> isn't the end of the world and its really no different than moving the
> task a little later anyway.
>
> task_group() should complain about this if you compile a kernel with
> CONFIG_PROVE_RCU.


Yeah, wrapping it around rcu_read_lock locks fine.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/