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Date: Sun, 8 May 2022 14:58:30 +0900
From: Hyeonggon Yoo <42.hyeyoo@...il.com>
To: Muchun Song <songmuchun@...edance.com>
Cc: cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
iamjoonsoo.kim@....com, akpm@...ux-foundation.org, vbabka@...e.cz,
roman.gushchin@...ux.dev, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, smuchun@...il.com
Subject: Re: [PATCH v2] mm: slab: optimize memcg_slab_free_hook()
On Fri, Apr 29, 2022 at 08:30:44PM +0800, Muchun Song wrote:
> Most callers of memcg_slab_free_hook() already know the slab, which could
> be passed to memcg_slab_free_hook() directly to reduce the overhead of an
> another call of virt_to_slab(). For bulk freeing of objects, the call of
> slab_objcgs() in the loop in memcg_slab_free_hook() is redundant as well.
> Rework memcg_slab_free_hook() and build_detached_freelist() to reduce
> those unnecessary overhead and make memcg_slab_free_hook() can handle bulk
> freeing in slab_free().
>
> Move the calling site of memcg_slab_free_hook() from do_slab_free() to
> slab_free() for slub to make the code clearer since the logic is weird
> (e.g. the caller need to judge whether it needs to call
> memcg_slab_free_hook()). It is easy to make mistakes like missing calling
> of memcg_slab_free_hook() like fixes of:
>
> commit d1b2cf6cb84a ("mm: memcg/slab: uncharge during kmem_cache_free_bulk()")
> commit ae085d7f9365 ("mm: kfence: fix missing objcg housekeeping for SLAB")
>
> This optimization is mainly for bulk objects freeing. The following numbers
> is shown for 16-object freeing.
>
> before after
> kmem_cache_free_bulk: ~430 ns ~400 ns
>
> The overhead is reduced by about 7% for 16-object freeing.
>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
> v2:
> - Add numbers to commit log.
>
> mm/slab.c | 4 ++--
> mm/slab.h | 30 ++++++++---------------------
> mm/slub.c | 66 +++++++++++++++++++++------------------------------------------
> 3 files changed, 32 insertions(+), 68 deletions(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index f8cd00f4ba13..2174962055ae 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3406,9 +3406,10 @@ static __always_inline void __cache_free(struct kmem_cache *cachep, void *objp,
> {
> bool init;
>
> + memcg_slab_free_hook(cachep, virt_to_slab(objp), &objp, 1);
> +
> if (is_kfence_address(objp)) {
> kmemleak_free_recursive(objp, cachep->flags);
> - memcg_slab_free_hook(cachep, &objp, 1);
> __kfence_free(objp);
> return;
> }
> @@ -3441,7 +3442,6 @@ void ___cache_free(struct kmem_cache *cachep, void *objp,
> check_irq_off();
> kmemleak_free_recursive(objp, cachep->flags);
> objp = cache_free_debugcheck(cachep, objp, caller);
> - memcg_slab_free_hook(cachep, &objp, 1);
>
SLAB part just looks fine.
> /*
> * Skip calling cache_free_alien() when the platform is not numa.
> diff --git a/mm/slab.h b/mm/slab.h
> index db9fb5c8dae7..a8d5eb1c323f 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -547,36 +547,22 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
> obj_cgroup_put(objcg);
> }
>
> -static inline void memcg_slab_free_hook(struct kmem_cache *s_orig,
> +static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
> void **p, int objects)
> {
> - struct kmem_cache *s;
> struct obj_cgroup **objcgs;
> - struct obj_cgroup *objcg;
> - struct slab *slab;
> - unsigned int off;
> int i;
>
> if (!memcg_kmem_enabled())
> return;
>
> - for (i = 0; i < objects; i++) {
> - if (unlikely(!p[i]))
> - continue;
> -
> - slab = virt_to_slab(p[i]);
> - /* we could be given a kmalloc_large() object, skip those */
> - if (!slab)
> - continue;
> -
Oh, memcg_slab_free_hook() no longer takes kmalloc_large() object becuase
such objects does not have corresponding slab.
> - objcgs = slab_objcgs(slab);
> - if (!objcgs)
> - continue;
> + objcgs = slab_objcgs(slab);
> + if (!objcgs)
> + return;
Now all objects in void **p should be in same slab,
so that we can remove some redundant part of loops that finds slab.
(virt_to_slab() and slab_objcgs())
> - if (!s_orig)
> - s = slab->slab_cache;
> - else
> - s = s_orig;
> + for (i = 0; i < objects; i++) {
> + struct obj_cgroup *objcg;
> + unsigned int off;
>
> off = obj_to_index(s, slab, p[i]);
> objcg = objcgs[off];
> @@ -628,7 +614,7 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
> {
> }
>
> -static inline void memcg_slab_free_hook(struct kmem_cache *s,
> +static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
> void **p, int objects)
> {
> }
> diff --git a/mm/slub.c b/mm/slub.c
> index 1f699ddfff7f..3794afe32b5f 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3435,9 +3435,6 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
> struct kmem_cache_cpu *c;
> unsigned long tid;
>
> - /* memcg_slab_free_hook() is already called for bulk free. */
> - if (!tail)
> - memcg_slab_free_hook(s, &head, 1);
> redo:
> /*
> * Determine the currently cpus per cpu slab.
> @@ -3497,9 +3494,10 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
> }
>
> static __always_inline void slab_free(struct kmem_cache *s, struct slab *slab,
> - void *head, void *tail, int cnt,
> + void *head, void *tail, void **p, int cnt,
> unsigned long addr)
> {
> + memcg_slab_free_hook(s, slab, p, cnt);
> /*
> * With KASAN enabled slab_free_freelist_hook modifies the freelist
> * to remove objects, whose reuse must be delayed.
> @@ -3521,7 +3519,7 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
> if (!s)
> return;
> trace_kmem_cache_free(_RET_IP_, x, s->name);
> - slab_free(s, virt_to_slab(x), x, NULL, 1, _RET_IP_);
> + slab_free(s, virt_to_slab(x), x, NULL, &x, 1, _RET_IP_);
> }
memcg slab free hook is moved from do_slab_free() to slab_free().
> EXPORT_SYMBOL(kmem_cache_free);
>
> @@ -3562,79 +3560,59 @@ static inline
> int build_detached_freelist(struct kmem_cache *s, size_t size,
> void **p, struct detached_freelist *df)
> {
> - size_t first_skipped_index = 0;
> int lookahead = 3;
> void *object;
> struct folio *folio;
> - struct slab *slab;
> -
> - /* Always re-init detached_freelist */
> - df->slab = NULL;
> -
> - do {
> - object = p[--size];
> - /* Do we need !ZERO_OR_NULL_PTR(object) here? (for kfree) */
> - } while (!object && size);
> -
> - if (!object)
> - return 0;
Now it does not mark p[size] = NULL as processed
> + size_t same;
>
> + object = p[--size];
> folio = virt_to_folio(object);
> if (!s) {
> /* Handle kalloc'ed objects */
> if (unlikely(!folio_test_slab(folio))) {
> free_large_kmalloc(folio, object);
> - p[size] = NULL; /* mark object processed */
> + df->slab = NULL;
> return size;
This part looks good too as kmem_cache_free_bulk() just skips if
df->slab == NULL.
> }
> /* Derive kmem_cache from object */
> - slab = folio_slab(folio);
> - df->s = slab->slab_cache;
> + df->slab = folio_slab(folio);
> + df->s = df->slab->slab_cache;
> } else {
> - slab = folio_slab(folio);
> + df->slab = folio_slab(folio);
> df->s = cache_from_obj(s, object); /* Support for memcg */
> }
>
> - if (is_kfence_address(object)) {
> - slab_free_hook(df->s, object, false);
> - __kfence_free(object);
> - p[size] = NULL; /* mark object processed */
> - return size;
> - }
> -
> /* Start new detached freelist */
> - df->slab = slab;
> - set_freepointer(df->s, object, NULL);
> df->tail = object;
> df->freelist = object;
> - p[size] = NULL; /* mark object processed */
> df->cnt = 1;
>
> + if (is_kfence_address(object))
> + return size;
> +
and kfence too is freed in slab_free() using detached freelist after
this patch.
> + set_freepointer(df->s, object, NULL);
> +
> + same = size;
> while (size) {
> object = p[--size];
> - if (!object)
> - continue; /* Skip processed objects */
> -
> /* df->slab is always set at this point */
> if (df->slab == virt_to_slab(object)) {
> /* Opportunity build freelist */
> set_freepointer(df->s, object, df->freelist);
> df->freelist = object;
> df->cnt++;
> - p[size] = NULL; /* mark object processed */
> -
> + same--;
> + if (size != same)
> + swap(p[size], p[same]);
> continue;
Now that SLUB does not mark processed objects as p[size] = NULL,
SLUB should put objects in same slab together, so swap() to put them
together.
> }
>
> /* Limit look ahead search */
> if (!--lookahead)
> break;
> -
> - if (!first_skipped_index)
> - first_skipped_index = size + 1;
> }
>
> - return first_skipped_index;
> + return same;
> }
same is last index of freed object in previous step,
so that should not be problem if next step treat that
as 'size' of array.
>
> /* Note that interrupts must be enabled when calling this function. */
> @@ -3643,7 +3621,6 @@ void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
> if (WARN_ON(!size))
> return;
>
> - memcg_slab_free_hook(s, p, size);
> do {
> struct detached_freelist df;
>
> @@ -3651,7 +3628,8 @@ void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
> if (!df.slab)
> continue;
>
> - slab_free(df.s, df.slab, df.freelist, df.tail, df.cnt, _RET_IP_);
> + slab_free(df.s, df.slab, df.freelist, df.tail, &p[size], df.cnt,
> + _RET_IP_);
> } while (likely(size));
> }
> EXPORT_SYMBOL(kmem_cache_free_bulk);
> @@ -4554,7 +4532,7 @@ void kfree(const void *x)
> return;
> }
> slab = folio_slab(folio);
> - slab_free(slab->slab_cache, slab, object, NULL, 1, _RET_IP_);
> + slab_free(slab->slab_cache, slab, object, NULL, &object, 1, _RET_IP_);
> }
> EXPORT_SYMBOL(kfree);
>
> --
> 2.11.0
>
>
Nice improvements,
Looks good to me.
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
--
Thanks,
Hyeonggon
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