Messages in this thread | | | From | Shakeel Butt <> | Date | Tue, 13 Mar 2018 10:55:18 -0700 | Subject | Re: [PATCH v3 1/2] mm: memcg: remote memcg charging for kmem allocations |
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On Tue, Mar 13, 2018 at 6:49 AM, Michal Hocko <mhocko@kernel.org> wrote: > On Wed 21-02-18 14:37:56, Shakeel Butt wrote: > [...] >> +#ifdef CONFIG_MEMCG >> +static inline struct mem_cgroup *memalloc_memcg_save(struct mem_cgroup *memcg) >> +{ >> + struct mem_cgroup *old_memcg = current->target_memcg; >> + current->target_memcg = memcg; >> + return old_memcg; >> +} > > So you are relying that the caller will handle the reference counting > properly? I do not think this is a good idea.
For the fsnotify use-case, this assumption makes sense as fsnotify has an abstraction of fsnotify_group which is created by the person/process interested in the events and thus can be used to hold the reference to the person/process's memcg. Another use-case I have in mind is the filesystem mount. Basically attaching a mount with a memcg and thus all user pages and kmem allocations (inodes, dentries) for that mount will be charged to the attached memcg. In this use-case the super_block is the perfect structure to hold the reference to the memcg.
If in future we find a use-case where this assumption does not make sense we can evolve the API and since this is kernel internal API, it should not be hard to evolve.
> Also do we need some kind > of debugging facility to detect unbalanced save/restore scopes? >
I am not sure, I didn't find other similar patterns (like PF_MEMALLOC) having debugging facility. Maybe we can add such debugging facility when we find more users other than kmalloc & kmem_cache_alloc. Vmalloc may be one but I could not think of a use-case for vmalloc for remote charging, so, no need to add more code at this time.
> [...] >> @@ -2260,7 +2269,10 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep) >> if (current->memcg_kmem_skip_account) >> return cachep; >> >> - memcg = get_mem_cgroup_from_mm(current->mm); >> + if (current->target_memcg) >> + memcg = get_mem_cgroup(current->target_memcg); >> + if (!memcg) >> + memcg = get_mem_cgroup_from_mm(current->mm); >> kmemcg_id = READ_ONCE(memcg->kmemcg_id); >> if (kmemcg_id < 0) >> goto out; > > You are also adding one branch for _each_ charge path even though the > usecase is rather limited. >
I understand the concern but the charging path, IMO, is much complex than just one or couple of additional branches. I can run a simple microbenchmark to see if there is anything noticeable here.
> I will have to think about this approach more. It is clearly less code > than your previous attempt but I cannot say I would be really impressed. >
Thanks for your time.
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