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SubjectRe: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators
On Wed 12-12-12 09:57:56, Ying Han wrote:
> On Wed, Dec 12, 2012 at 12:55 AM, Michal Hocko <mhocko@suse.cz> wrote:
> > On Tue 11-12-12 14:43:37, Ying Han wrote:
> >> On Tue, Dec 11, 2012 at 8:15 AM, Michal Hocko <mhocko@suse.cz> wrote:
> >> > On Tue 11-12-12 16:50:25, Michal Hocko wrote:
> >> >> On Sun 09-12-12 08:59:54, Ying Han wrote:
> >> >> > On Mon, Nov 26, 2012 at 10:47 AM, Michal Hocko <mhocko@suse.cz> wrote:
> >> >> [...]
> >> >> > > + /*
> >> >> > > + * Even if we found a group we have to make sure it is alive.
> >> >> > > + * css && !memcg means that the groups should be skipped and
> >> >> > > + * we should continue the tree walk.
> >> >> > > + * last_visited css is safe to use because it is protected by
> >> >> > > + * css_get and the tree walk is rcu safe.
> >> >> > > + */
> >> >> > > + if (css == &root->css || (css && css_tryget(css)))
> >> >> > > + memcg = mem_cgroup_from_css(css);
> >> >> > >
> >> >> > > if (reclaim) {
> >> >> > > - iter->position = id;
> >> >> > > + struct mem_cgroup *curr = memcg;
> >> >> > > +
> >> >> > > + if (last_visited)
> >> >> > > + css_put(&last_visited->css);
> >> >> > > +
> >> >> > > + if (css && !memcg)
> >> >> > > + curr = mem_cgroup_from_css(css);
> >> >> >
> >> >> > In this case, the css_tryget() failed which implies the css is on the
> >> >> > way to be removed. (refcnt ==0) If so, why it is safe to call
> >> >> > css_get() directly on it below? It seems not preventing the css to be
> >> >> > removed by doing so.
> >> >>
> >> >> Well, I do not remember exactly but I guess the code is meant to say
> >> >> that we need to store a half-dead memcg because the loop has to be
> >> >> retried. As we are under RCU hood it is just half dead.
> >> >> Now that you brought this up I think this is not safe as well because
> >> >> another thread could have seen the cached value while we tried to retry
> >> >> and his RCU is not protecting the group anymore.
> >> >
> >> > Hmm, thinking about it some more, it _is_ be safe in the end.
> >> >
> >> > We are safe because we are under RCU. And even if somebody else looked
> >> > at the half-dead memcg from iter->last_visited it cannot disappear
> >> > because the current one will retry without dropping RCU so the grace
> >> > period couldn't have been finished.
> >> >
> >> > CPU0 CPU1
> >> > rcu_read_lock() rcu_read_lock()
> >> > while(!memcg) { while(!memcg)
> >> > [...]
> >> > spin_lock(&iter->iter_lock)
> >> > [...]
> >> > if (css == &root->css ||
> >> > (css && css_tryget(css)))
> >> > memcg = mem_cgroup_from_css(css)
> >> > [...]
> >> > if (css && !memcg)
> >> > curr = mem_cgroup_from_css(css)
> >> > if (curr)
> >> > css_get(curr);
> >> > spin_unlock(&iter->iter_lock)
> >> > spin_lock(&iter->iter_lock)
> >> > /* sees the half dead memcg but its cgroup is still valid */
> >> > [...]
> >> > spin_unlock(&iter->iter_lock)
> >> > /* we do retry */
> >> > }
> >> > rcu_read_unlock()
> >> >
> >> > so the css_get will just helps to prevent from further code obfuscation.
> >> >
> >> > Makes sense? The code gets much simplified later in the series,
> >> > fortunately.
> >>
> >> My understanding on this is that we should never call css_get()
> >> without calling css_tryget() and it succeed.
> >
> > Hmm, what would be the point of using css_get then?
>
> Only css_tryget() will fail if the cgroup is under removal, but not
> css_get(). AFAIK there is logic in cgroup_rmdir() rely on that. (The
> CSS_DEACT_BIAS will block new css_tryget(), and then fail all further
> css_get(). )
>
> >
> >> Whether or not it is *safe* to do so, that seems conflicts with the
> >> assumption of the cgroup_rmdir().
> >>
> >> I would rather make the change to do the retry after css_tryget()
> >> failed. The patch I have on my local tree:
> >
> > OK, I am not against, the retry is just nicer and that is the reason
> > I changed that in the follow up patch. Just note that this is an
> > intermediate patch and the code is changed significantly in the later
> > patches so the question is whether it is worth changing that.
> > This surely couldn't have caused your testing issue, right?
>
> I haven't tested separately, but the retry logic +
> mem_cgroup_iter_break() change cure my testcase.

I bet that what you are seeing is the stale cgroup due to cached
memcg.
Retry logic doesn't help with that as the elevated ref count is just
temporal but your mem_cgroup_iter_break change might help for targeted
reclaim as it doesn't leave the memcg in last_visited (it still
shouldn't help the global reclaim case though). But this is not correct
because it will break the concurent reclaim as I mentioned previously.

I will try to post my pending patch to heal this ASAP.

Thanks
--
Michal Hocko
SUSE Labs


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