Re: [PATCH 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab caches with SLAB_ACCOUNT

[Roman Gushchin <guro@fb.com>]

On Thu, Nov 12, 2020 at 11:23:03AM -0500, Johannes Weiner wrote:
> On Tue, Nov 10, 2020 at 11:57:53AM -0800, Roman Gushchin wrote:
> > 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.
> > 
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> That's a nice optimization!
> 
> Some comments inline:
> 
> > @@ -485,14 +485,20 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
> >   * 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)
> > +				   struct obj_cgroup **objcgs, bool atomic)
> 
> bool parameters make callsites pretty hard to understand - unless the
> function interface obviously has two modes (read vs write etc.), which
> isn't the case here.
> 
> > -	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs |
> > -			MEMCG_DATA_OBJCGS);
> > +	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)
> > @@ -506,7 +512,7 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
> >  }
> >  
> >  static inline bool set_page_objcgs(struct page *page,
> > -					struct obj_cgroup **objcgs)
> > +				   struct obj_cgroup **objcgs, bool atomic)
> >  {
> >  	return true;
> >  }
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 69a2893a6455..37bffd336235 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2874,7 +2874,7 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg)
> >  
> >  #ifdef CONFIG_MEMCG_KMEM
> >  int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
> > -				 gfp_t gfp)
> > +				 gfp_t gfp, bool atomic)
> >  {
> >  	unsigned int objects = objs_per_slab_page(s, page);
> >  	void *vec;
> > @@ -2884,7 +2884,7 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
> >  	if (!vec)
> >  		return -ENOMEM;
> >  
> > -	if (!set_page_objcgs(page, vec))
> > +	if (!set_page_objcgs(page, vec, atomic))
> >  		kfree(vec);
> >  	else
> >  		kmemleak_not_leak(vec);
> 
> The life of page->memcg_data and this optimization could use a central
> comment somewhere, because it's hard to understand what's going on
> from the code alone. This function here seems like a good place?
> 
> I don't see a way to eliminate the bool on the allocation function,
> but maybe it could be more descriptive. Maybe bool slab_account?
> 
> set_page_objcgs() can be inlined at this point. It made some sense to
> abstract away the atomics with setter and matching getter, but with a
> non-atomic mode, inlining makes things clearer and allows for better
> in-place documentation in the sole callsite.

I agree, inlining makes sense here.

I'd prefer not to spill details about SLAB_ACCOUNT into memcg_alloc_page_obj_cgroups().
What's important there is only whether we have an exclusive access to the page or not.

How about the version below?

Thanks!

--

From 8b28d91475d54c552e503e66f169e1e00475c856 Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@fb.com>
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

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/memcontrol.h | 19 -------------------
 mm/memcontrol.c            | 23 +++++++++++++++++++----
 mm/slab.c                  |  2 +-
 mm/slab.h                  | 14 ++++++++++----
 4 files changed, 30 insertions(+), 28 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 20108e426f84..bfcdcb2d2976 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -481,19 +481,6 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
 	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
- *
- * Atomically associates a page with a vector of object cgroups.
- */
-static inline bool set_page_objcgs(struct page *page,
-					struct obj_cgroup **objcgs)
-{
-	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs |
-			MEMCG_DATA_OBJCGS);
-}
 #else
 static inline struct obj_cgroup **page_objcgs(struct page *page)
 {
@@ -504,12 +491,6 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
 {
 	return NULL;
 }
-
-static inline bool set_page_objcgs(struct page *page,
-					struct obj_cgroup **objcgs)
-{
-	return true;
-}
 #endif
 
 static __always_inline bool memcg_stat_item_in_bytes(int idx)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 69a2893a6455..6ca8c23d7816 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2874,9 +2874,10 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg)
 
 #ifdef CONFIG_MEMCG_KMEM
 int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
-				 gfp_t gfp)
+				 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,
@@ -2884,11 +2885,25 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
 	if (!vec)
 		return -ENOMEM;
 
-	if (!set_page_objcgs(page, vec))
+	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;
 }
 
diff --git a/mm/slab.c b/mm/slab.c
index c0ea4b1c7088..df0299e1d0b9 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1380,7 +1380,7 @@ static struct page *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
 		return NULL;
 	}
 
-	account_slab_page(page, cachep->gfporder, cachep);
+	account_slab_page(page, cachep->gfporder, cachep, flags);
 	__SetPageSlab(page);
 	/* Record if ALLOC_NO_WATERMARKS was set when allocating the slab */
 	if (sk_memalloc_socks() && page_is_pfmemalloc(page))
diff --git a/mm/slab.h b/mm/slab.h
index c73050654b8a..59aeb0d9f11b 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -240,7 +240,7 @@ static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t fla
 
 #ifdef CONFIG_MEMCG_KMEM
 int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
-				 gfp_t gfp);
+				 gfp_t gfp, bool new_page);
 
 static inline void memcg_free_page_obj_cgroups(struct page *page)
 {
@@ -307,7 +307,8 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
 			page = virt_to_head_page(p[i]);
 
 			if (!page_objcgs(page) &&
-			    memcg_alloc_page_obj_cgroups(page, s, flags)) {
+			    memcg_alloc_page_obj_cgroups(page, s, flags,
+							 false)) {
 				obj_cgroup_uncharge(objcg, obj_full_size(s));
 				continue;
 			}
@@ -371,7 +372,8 @@ static inline struct mem_cgroup *memcg_from_slab_obj(void *ptr)
 }
 
 static inline int memcg_alloc_page_obj_cgroups(struct page *page,
-					       struct kmem_cache *s, gfp_t gfp)
+					       struct kmem_cache *s, gfp_t gfp,
+					       bool new_page)
 {
 	return 0;
 }
@@ -412,8 +414,12 @@ static inline struct kmem_cache *virt_to_cache(const void *obj)
 }
 
 static __always_inline void account_slab_page(struct page *page, int order,
-					      struct kmem_cache *s)
+					      struct kmem_cache *s,
+					      gfp_t gfp)
 {
+	if (memcg_kmem_enabled() && (s->flags & SLAB_ACCOUNT))
+		memcg_alloc_page_obj_cgroups(page, s, gfp, true);
+
 	mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
 			    PAGE_SIZE << order);
 }
-- 
2.26.2