Re: [PATCH] mm: slab: optimize memcg_slab_free_hook()

[Roman Gushchin <roman.gushchin@linux.dev>]

On Mon, Apr 25, 2022 at 11:54:06AM +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")
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/slab.c |  4 ++--
>  mm/slab.h | 30 ++++++++---------------------
>  mm/slub.c | 66 +++++++++++++++++++++------------------------------------------
>  3 files changed, 32 insertions(+), 68 deletions(-)

Overall it looks like a really nice cleanup!

One nit below.

> 
> diff --git a/mm/slab.c b/mm/slab.c
> index a301f266efd1..e868b4af4346 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3407,9 +3407,10 @@ 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;
>  	}
> @@ -3442,7 +3443,6 @@ 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);
>  
>  	/*
>  	 * 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;
> -
> -		objcgs = slab_objcgs(slab);
> -		if (!objcgs)
> -			continue;
> +	objcgs = slab_objcgs(slab);
> +	if (!objcgs)
> +		return;
>  
> -		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 6dc703488d30..86c50eb6c670 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3437,9 +3437,6 @@ 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.
> @@ -3499,9 +3496,10 @@ 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.
> @@ -3523,7 +3521,7 @@ 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_);
>  }
>  EXPORT_SYMBOL(kmem_cache_free);
>  
> @@ -3564,79 +3562,59 @@ 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;
> +	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;
>  		}
>  		/* 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;
> +
> +	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;
>  		}
>  
>  		/* Limit look ahead search */
>  		if (!--lookahead)
>  			break;
> -
> -		if (!first_skipped_index)
> -			first_skipped_index = size + 1;
>  	}
>  
> -	return first_skipped_index;
> +	return same;
>  }
>  
>  /* Note that interrupts must be enabled when calling this function. */
> @@ -3645,7 +3623,6 @@ 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;
>  
> @@ -3653,7 +3630,8 @@ 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));

As I understand, it might result in more memcg_slab_free_hook() calls, right?
I guess you might want to include some benchmark results, which actually
might look very good because of optimizations above.

Thanks!