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@xxxxxx>
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.
How about something like this?
vec = kcalloc(...);
memcg_data = (unsigned long)vec | MEMCG_DATA_OBJCGS;
/*
* Set up the objcg vector for the page.
*
* When only some objects in a slab are tracked (think GFP_ACCOUNT
* kmalloc allocations), the objcg vector is set up when the first
* tracked object in the slab page is allocated. Multiple concurrent
* slab allocations can race to this, so synchronization is required.
*
* When SLAB_ACCOUNT is set on the cache, however, all objects in the
* slab page will be tracked, and the vector is allocated along with
* the page itself, while it's still exclusive; no atomics necessary.
*/
if (slab_account) {
page->memcg_data = memcg_data;
} else {
if (cmpxchg(&page->memcg_data, 0, memcg_data)) {
/* Somebody else beat us, use their vec */
kfree(vec);
return 0;
}
}
kmemleak_not_leak(vec);
return 0;
