On Mon, Jun 22, 2020 at 02:28:54PM -0700, Shakeel Butt wrote: > On Mon, Jun 22, 2020 at 2:15 PM Roman Gushchin <g...@fb.com> wrote: > > > > On Mon, Jun 22, 2020 at 02:04:29PM -0700, Shakeel Butt wrote: > > > On Mon, Jun 22, 2020 at 1:37 PM Roman Gushchin <g...@fb.com> wrote: > > > > > > > > On Mon, Jun 22, 2020 at 12:21:28PM -0700, Shakeel Butt wrote: > > > > > On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <g...@fb.com> wrote: > > > > > > > > > > > > Instead of having two sets of kmem_caches: one for system-wide and > > > > > > non-accounted allocations and the second one shared by all accounted > > > > > > allocations, we can use just one. > > > > > > > > > > > > The idea is simple: space for obj_cgroup metadata can be allocated > > > > > > on demand and filled only for accounted allocations. > > > > > > > > > > > > It allows to remove a bunch of code which is required to handle > > > > > > kmem_cache clones for accounted allocations. There is no more need > > > > > > to create them, accumulate statistics, propagate attributes, etc. > > > > > > It's a quite significant simplification. > > > > > > > > > > > > Also, because the total number of slab_caches is reduced almost > > > > > > twice > > > > > > (not all kmem_caches have a memcg clone), some additional memory > > > > > > savings are expected. On my devvm it additionally saves about 3.5% > > > > > > of slab memory. > > > > > > > > > > > > Suggested-by: Johannes Weiner <han...@cmpxchg.org> > > > > > > Signed-off-by: Roman Gushchin <g...@fb.com> > > > > > > Reviewed-by: Vlastimil Babka <vba...@suse.cz> > > > > > > --- > > > > > [snip] > > > > > > static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s, > > > > > > struct obj_cgroup > > > > > > *objcg, > > > > > > - size_t size, void **p) > > > > > > + gfp_t flags, size_t > > > > > > size, > > > > > > + void **p) > > > > > > { > > > > > > struct page *page; > > > > > > unsigned long off; > > > > > > size_t i; > > > > > > > > > > > > + if (!objcg) > > > > > > + return; > > > > > > + > > > > > > + flags &= ~__GFP_ACCOUNT; > > > > > > for (i = 0; i < size; i++) { > > > > > > if (likely(p[i])) { > > > > > > page = virt_to_head_page(p[i]); > > > > > > + > > > > > > + if (!page_has_obj_cgroups(page) && > > > > > > > > > > The page is already linked into the kmem_cache, don't you need > > > > > synchronization for memcg_alloc_page_obj_cgroups(). > > > > > > > > Hm, yes, in theory we need it. I guess the reason behind why I've never > > > > seen any issues > > > > here is the SLUB percpu partial list. > > > > > > > > So in theory we need something like: > > > > > > > > diff --git a/mm/slab.h b/mm/slab.h > > > > index 0a31600a0f5c..44bf57815816 100644 > > > > --- a/mm/slab.h > > > > +++ b/mm/slab.h > > > > @@ -237,7 +237,10 @@ static inline int > > > > memcg_alloc_page_obj_cgroups(struct page *page, > > > > if (!vec) > > > > return -ENOMEM; > > > > > > > > - page->obj_cgroups = (struct obj_cgroup **) ((unsigned long)vec > > > > | 0x1UL); > > > > + if (cmpxchg(&page->obj_cgroups, 0, > > > > + (struct obj_cgroup **) ((unsigned long)vec | > > > > 0x1UL))) > > > > + kfree(vec); > > > > + > > > > return 0; > > > > } > > > > > > > > > > > > But I wonder if we might put it under #ifdef CONFIG_SLAB? > > > > Or any other ideas how to make it less expensive? > > > > > > > > > What's the reason to remove this from charge_slab_page()? > > > > > > > > Because at charge_slab_page() we don't know if we'll ever need > > > > page->obj_cgroups. Some caches might have only few or even zero > > > > accounted objects. > > > > > > > > > > If slab_pre_alloc_hook() returns a non-NULL objcg then we definitely > > > need page->obj_cgroups. The charge_slab_page() happens between > > > slab_pre_alloc_hook() & slab_post_alloc_hook(), so, we should be able > > > to tell if page->obj_cgroups is needed. > > > > Yes, but the opposite is not always true: we can reuse the existing page > > without allocated page->obj_cgroups. In this case charge_slab_page() is > > not involved at all. > > > > Hmm yeah, you are right. I missed that. > > > > > Or do you mean that we can minimize the amount of required synchronization > > by allocating some obj_cgroups vectors from charge_slab_page()? > > One optimization would be to always pre-allocate page->obj_cgroups for > kmem_caches with SLAB_ACCOUNT.
Even this is not completely memory overhead-free, because processes belonging to the root cgroup and kthreads might allocate from such cache. Anyway, I think I'll go with cmpxchg() for now and will think about possible optimizations later. Because the allocation happens only once per the lifetime of a slab page, and is very unlikely racing with a concurrent one on the same page, the penalty shouldn't be that big. Thanks!