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Message-ID: <20201113001926.GA2934489@carbon.dhcp.thefacebook.com>
Date: Thu, 12 Nov 2020 16:19:26 -0800
From: Roman Gushchin <guro@...com>
To: Johannes Weiner <hannes@...xchg.org>
CC: Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
Shakeel Butt <shakeelb@...gle.com>,
Michal Hocko <mhocko@...nel.org>,
Christoph Lameter <cl@...ux.com>,
<linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH 2/2] mm: memcg/slab: pre-allocate obj_cgroups for slab
caches with SLAB_ACCOUNT
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@...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@...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@...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
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