Re: [RFC 4/5] memcg: make sure that memcg is not offline when charging

[Michal Hocko <mhocko@suse.cz>]

On Thu 30-01-14 12:29:06, Johannes Weiner wrote:
> On Tue, Dec 17, 2013 at 04:45:29PM +0100, Michal Hocko wrote:
> > The current charge path might race with memcg offlining because holding
> > css reference doesn't stop css offline. As a result res counter might be
> > charged after mem_cgroup_reparent_charges (called from memcg css_offline
> > callback) and so the charge would never be freed. This has been worked
> > around by 96f1c58d8534 (mm: memcg: fix race condition between memcg
> > teardown and swapin) which tries to catch such a leaked charges later
> > during css_free. It is more optimal to heal this race in the long term
> > though.
> 
> We already deal with the race, so IMO the only outstanding improvement
> is to take advantage of the teardown synchronization provided by the
> cgroup core and get rid of our one-liner workaround in .css_free.

I am not sure I am following you here. Which teardown synchronization do
you have in mind? rcu_read_lock & css_tryget?

> > In order to make this raceless we would need to hold rcu_read_lock since
> > css_tryget until res_counter_charge. This is not so easy unfortunately
> > because mem_cgroup_do_charge might sleep so we would need to do drop rcu
> > lock and do css_tryget tricks after each reclaim.
> 
> Yes, why not?

Although css_tryget is cheap these days I thought that a simple flag
check would be even heaper in this hot path. Changing the patch to use
css_tryget rather than offline check is trivial if you really think it
is better?

Btw. I plan to repost the series as soon as Andrew releases his mmotm
tree.

Thanks
-- 
Michal Hocko
SUSE Labs

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