On Thu, 12 Nov 2020 16:19:26 -0800 Roman Gushchin <guro@xxxxxx> wrote: > >From 8b28d91475d54c552e503e66f169e1e00475c856 Mon Sep 17 00:00:00 2001 > From: Roman Gushchin <guro@xxxxxx> > Date: Wed, 16 Sep 2020 15:43:48 -0700 > Subject: [PATCH v2 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab > caches with SLAB_ACCOUNT > > In general it's unknown in advance if a slab page will contain > accounted objects or not. In order to avoid memory waste, an > obj_cgroup vector is allocated dynamically when a need to account > of a new object arises. Such approach is memory efficient, but > requires an expensive cmpxchg() to set up the memcg/objcgs pointer, > because an allocation can race with a different allocation on another > cpu. > > But in some common cases it's known for sure that a slab page will > contain accounted objects: if the page belongs to a slab cache with a > SLAB_ACCOUNT flag set. It includes such popular objects like > vm_area_struct, anon_vma, task_struct, etc. > > In such cases we can pre-allocate the objcgs vector and simple assign > it to the page without any atomic operations, because at this early > stage the page is not visible to anyone else. > > v2: inline set_page_objcgs() and add some comments, by Johannes Had me confused! I was looking for the inlined function set_page_objcgs(). I think "open-code" is a better term here than "inline". Here's the -fix: From: Roman Gushchin <guro@xxxxxx> Subject: mm-memcg-slab-pre-allocate-obj_cgroups-for-slab-caches-with-slab_account-v2 open-code set_page_objcgs() and add some comments, by Johannes Link: https://lkml.kernel.org/r/20201113001926.GA2934489@xxxxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Roman Gushchin <guro@xxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/memcontrol.h | 25 ------------------------- mm/memcontrol.c | 23 +++++++++++++++++++---- mm/slab.h | 8 ++++---- 3 files changed, 23 insertions(+), 33 deletions(-) --- a/include/linux/memcontrol.h~mm-memcg-slab-pre-allocate-obj_cgroups-for-slab-caches-with-slab_account-v2 +++ a/include/linux/memcontrol.h @@ -480,25 +480,6 @@ static inline struct obj_cgroup **page_o return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK); } -/* - * set_page_objcgs - associate a page with a object cgroups vector - * @page: a pointer to the page struct - * @objcgs: a pointer to the object cgroups vector - * @atomic: save the value atomically - * - * Atomically associates a page with a vector of object cgroups. - */ -static inline bool set_page_objcgs(struct page *page, - struct obj_cgroup **objcgs, bool atomic) -{ - unsigned long memcg_data = (unsigned long) objcgs | MEMCG_DATA_OBJCGS; - - if (atomic) - return !cmpxchg(&page->memcg_data, 0, memcg_data); - - page->memcg_data = memcg_data; - return true; -} #else static inline struct obj_cgroup **page_objcgs(struct page *page) { @@ -509,12 +490,6 @@ static inline struct obj_cgroup **page_o { return NULL; } - -static inline bool set_page_objcgs(struct page *page, - struct obj_cgroup **objcgs, bool atomic) -{ - return true; -} #endif static __always_inline bool memcg_stat_item_in_bytes(int idx) --- a/mm/memcontrol.c~mm-memcg-slab-pre-allocate-obj_cgroups-for-slab-caches-with-slab_account-v2 +++ a/mm/memcontrol.c @@ -2879,9 +2879,10 @@ static void commit_charge(struct page *p #ifdef CONFIG_MEMCG_KMEM int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s, - gfp_t gfp, bool atomic) + gfp_t gfp, bool new_page) { unsigned int objects = objs_per_slab_page(s, page); + unsigned long memcg_data; void *vec; vec = kcalloc_node(objects, sizeof(struct obj_cgroup *), gfp, @@ -2889,11 +2890,25 @@ int memcg_alloc_page_obj_cgroups(struct if (!vec) return -ENOMEM; - if (!set_page_objcgs(page, vec, atomic)) + memcg_data = (unsigned long) vec | MEMCG_DATA_OBJCGS; + if (new_page) { + /* + * If the slab page is brand new and nobody can yet access + * it's memcg_data, no synchronization is required and + * memcg_data can be simply assigned. + */ + page->memcg_data = memcg_data; + } else if (cmpxchg(&page->memcg_data, 0, memcg_data)) { + /* + * If the slab page is already in use, somebody can allocate + * and assign obj_cgroups in parallel. In this case the existing + * objcg vector should be reused. + */ kfree(vec); - else - kmemleak_not_leak(vec); + return 0; + } + kmemleak_not_leak(vec); return 0; } --- a/mm/slab.h~mm-memcg-slab-pre-allocate-obj_cgroups-for-slab-caches-with-slab_account-v2 +++ a/mm/slab.h @@ -240,7 +240,7 @@ static inline bool kmem_cache_debug_flag #ifdef CONFIG_MEMCG_KMEM int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s, - gfp_t gfp, bool atomic); + gfp_t gfp, bool new_page); static inline void memcg_free_page_obj_cgroups(struct page *page) { @@ -308,7 +308,7 @@ static inline void memcg_slab_post_alloc if (!page_objcgs(page) && memcg_alloc_page_obj_cgroups(page, s, flags, - true)) { + false)) { obj_cgroup_uncharge(objcg, obj_full_size(s)); continue; } @@ -373,7 +373,7 @@ static inline struct mem_cgroup *memcg_f static inline int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s, gfp_t gfp, - bool atomic) + bool new_page) { return 0; } @@ -418,7 +418,7 @@ static __always_inline void account_slab gfp_t gfp) { if (memcg_kmem_enabled() && (s->flags & SLAB_ACCOUNT)) - memcg_alloc_page_obj_cgroups(page, s, gfp, false); + memcg_alloc_page_obj_cgroups(page, s, gfp, true); mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s), PAGE_SIZE << order); _
