On Wed 05-06-13 01:20:23, Tejun Heo wrote: > Hello, Michal. > > On Wed, Jun 05, 2013 at 09:30:23AM +0200, Michal Hocko wrote: > > > I don't really get that. As long as the amount is bound and the > > > overhead negligible / acceptable, why does it matter how long the > > > pinning persists? > > > > Because the amount is not bound either. Just create a hierarchy and > > trigger the hard limit and if you are careful enough you can always keep > > some of the children in the cached pointer (with css reference, if you > > will) and then release the hierarchy. You can do that repeatedly and > > leak considerable amount of memory. > > It's still bound, no? Each live memcg can only keep limited number of > cgroups cached, right? Assuming that they are cleaned up when the memcg is offlined then yes. > > > We aren't talking about something gigantic or can > > > > mem_cgroup is 888B now (depending on configuration). So I wouldn't call > > it negligible. > > Do you think that the number can actually grow harmful? Would you be > kind enough to share some calculations with me? Well, each intermediate node might pin up-to NR_NODES * NR_ZONES * NR_PRIORITY groups. You would need a big hierarchy to have chance to cache different groups so that it starts matter. The problem is the clean up though. It might be a simple object at the time when it never gets freed. So there _must_ be something that would release the css reference to free the associated resources. As I said this can be done either during css_offline or in a lazy fashion that we have currently. I really do not care much which way it is done. > > > In the off chance that this is a real problem, which I strongly doubt, > > > as I wrote to Johannes, we can implement extremely dumb cleanup > > > routine rather than this weak reference beast. > > > > That was my first version (https://lkml.org/lkml/2013/1/3/298) and > > Johannes didn't like. To be honest I do not care _much_ which way we go > > but we definitely cannot pin those objects for ever. > > I'll get to the barrier thread but really complex barrier dancing like > that is only justifiable in extremely hot paths a lot of people pay > attention to. It doesn't belong inside memcg proper. If the cached > amount is an actual concern, let's please implement a simple clean up > thing. All we need is a single delayed_work which scans the tree > periodically. And do what? css_try_get to find out whether the cached memcg is still alive. Sorry, I do not like it at all. I find it much better to clean up when the group is removed. Because doing things asynchronously just makes it more obscure. There is no reason to do such a thing on the background when we know _when_ to do the cleanup and that is definitely _not a hot path_. > Johannes, what do you think? > > Thanks. > > -- > tejun -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>