On 08/20/2012 05:36 PM, Kamezawa Hiroyuki wrote: > (2012/08/16 2:00), Glauber Costa wrote: >> On 08/15/2012 08:38 PM, Greg Thelen wrote: >>> On Wed, Aug 15 2012, Glauber Costa wrote: >>> >>>> On 08/14/2012 10:58 PM, Greg Thelen wrote: >>>>> On Mon, Aug 13 2012, Glauber Costa wrote: >>>>> >>>>>>>>> + WARN_ON(mem_cgroup_is_root(memcg)); >>>>>>>>> + size = (1 << order) << PAGE_SHIFT; >>>>>>>>> + memcg_uncharge_kmem(memcg, size); >>>>>>>>> + mem_cgroup_put(memcg); >>>>>>> Why do we need ref-counting here ? kmem res_counter cannot work as >>>>>>> reference ? >>>>>> This is of course the pair of the mem_cgroup_get() you commented on >>>>>> earlier. If we need one, we need the other. If we don't need one, we >>>>>> don't need the other =) >>>>>> >>>>>> The guarantee we're trying to give here is that the memcg >>>>>> structure will >>>>>> stay around while there are dangling charges to kmem, that we decided >>>>>> not to move (remember: moving it for the stack is simple, for the >>>>>> slab >>>>>> is very complicated and ill-defined, and I believe it is better to >>>>>> treat >>>>>> all kmem equally here) >>>>> >>>>> By keeping memcg structures hanging around until the last referring >>>>> kmem >>>>> page is uncharged do such zombie memcg each consume a css_id and thus >>>>> put pressure on the 64k css_id space? I imagine in pathological cases >>>>> this would prevent creation of new cgroups until these zombies are >>>>> dereferenced. >>>> >>>> Yes, but although this patch makes it more likely, it doesn't introduce >>>> that. If the tasks, for instance, grab a reference to the cgroup dentry >>>> in the filesystem (like their CWD, etc), they will also keep the cgroup >>>> around. >>> >>> Fair point. But this doesn't seems like a feature. It's probably not >>> needed initially, but what do you think about creating a >>> memcg_kernel_context structure which is allocated when memcg is >>> allocated? Kernel pages charged to a memcg would have >>> page_cgroup->mem_cgroup=memcg_kernel_context rather than memcg. This >>> would allow the mem_cgroup and its css_id to be deleted when the cgroup >>> is unlinked from cgroupfs while allowing for the active kernel pages to >>> continue pointing to a valid memcg_kernel_context. This would be a >>> reference counted structure much like you are doing with memcg. When a >>> memcg is deleted the memcg_kernel_context would be linked into its >>> surviving parent memcg. This would avoid needing to visit each kernel >>> page. >> >> You need more, you need at the res_counters to stay around as well. And >> probably other fields. >> >> So my fear here is that as you add fields to that structure, you can >> defeat a bit the goal of reducing memory consumption. Still leaves the >> css space, yes. But by doing this we can introduce some subtle bugs by >> having a field in the wrong structure. >> > > Hm, can't we free css_id and delete css structure from the css_id idr tree > when a memcg goes zombie ? > > Thanks, > -Kame Kame, I wrote a patch that does exactly that. Can you take a look? (I posted it already) I actually need to go back to it, because greg seems to be right saying that that will break things for memsw. But a simplified version may work. -- 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>