Re: [RFC PATCH v3 7/7] slub: Optimize deactivate_slab()

[Vlastimil Babka <vbabka@suse.cz>]

On 10/24/23 11:33, chengming.zhou@linux.dev wrote:
> From: Chengming Zhou <zhouchengming@bytedance.com>
> 
> Since the introduce of unfrozen slabs on cpu partial list, we don't
> need to synchronize the slab frozen state under the node list_lock.
> 
> The caller of deactivate_slab() and the caller of __slab_free() won't
> manipulate the slab list concurrently.
> 
> So we can get node list_lock in the last stage if we really need to
> manipulate the slab list in this path.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

Nice simplification!

> ---
>  mm/slub.c | 70 ++++++++++++++++++++-----------------------------------
>  1 file changed, 25 insertions(+), 45 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 486d44421432..64d550e415eb 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2449,10 +2449,8 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
>  static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>  			    void *freelist)
>  {
> -	enum slab_modes { M_NONE, M_PARTIAL, M_FREE, M_FULL_NOLIST };
>  	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
>  	int free_delta = 0;
> -	enum slab_modes mode = M_NONE;
>  	void *nextfree, *freelist_iter, *freelist_tail;
>  	int tail = DEACTIVATE_TO_HEAD;
>  	unsigned long flags = 0;
> @@ -2499,58 +2497,40 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>  	 * unfrozen and number of objects in the slab may have changed.
>  	 * Then release lock and retry cmpxchg again.
>  	 */

This comment above (including parts not visible here) should be updated as
there is no more list manipulation during unfreeze.

> -redo:
> -
> -	old.freelist = READ_ONCE(slab->freelist);
> -	old.counters = READ_ONCE(slab->counters);
> -	VM_BUG_ON(!old.frozen);
> -
> -	/* Determine target state of the slab */
> -	new.counters = old.counters;
> -	if (freelist_tail) {
> -		new.inuse -= free_delta;
> -		set_freepointer(s, freelist_tail, old.freelist);
> -		new.freelist = freelist;
> -	} else
> -		new.freelist = old.freelist;
> +	do {
> +		old.freelist = READ_ONCE(slab->freelist);
> +		old.counters = READ_ONCE(slab->counters);
> +		VM_BUG_ON(!old.frozen);
> +
> +		/* Determine target state of the slab */
> +		new.counters = old.counters;
> +		new.frozen = 0;
> +		if (freelist_tail) {
> +			new.inuse -= free_delta;
> +			set_freepointer(s, freelist_tail, old.freelist);
> +			new.freelist = freelist;
> +		} else
> +			new.freelist = old.freelist;

Per coding style we should have the else with { } even if it's one line, to
match the if branch. Since we touch the code that was previously violating
the style, we can fix up.

>  
> -	new.frozen = 0;
> +	} while (!slab_update_freelist(s, slab,
> +		old.freelist, old.counters,
> +		new.freelist, new.counters,
> +		"unfreezing slab"));
>  
> +	/*
> +	 * Stage three: Manipulate the slab list based on the updated state.
> +	 */
>  	if (!new.inuse && n->nr_partial >= s->min_partial) {
> -		mode = M_FREE;
> +		stat(s, DEACTIVATE_EMPTY);
> +		discard_slab(s, slab);
> +		stat(s, FREE_SLAB);
>  	} else if (new.freelist) {
> -		mode = M_PARTIAL;
> -		/*
> -		 * Taking the spinlock removes the possibility that
> -		 * acquire_slab() will see a slab that is frozen
> -		 */
>  		spin_lock_irqsave(&n->list_lock, flags);
> -	} else {
> -		mode = M_FULL_NOLIST;
> -	}
> -
> -
> -	if (!slab_update_freelist(s, slab,
> -				old.freelist, old.counters,
> -				new.freelist, new.counters,
> -				"unfreezing slab")) {
> -		if (mode == M_PARTIAL)
> -			spin_unlock_irqrestore(&n->list_lock, flags);
> -		goto redo;
> -	}
> -
> -
> -	if (mode == M_PARTIAL) {
>  		add_partial(n, slab, tail);
>  		spin_unlock_irqrestore(&n->list_lock, flags);
>  		stat(s, tail);
> -	} else if (mode == M_FREE) {
> -		stat(s, DEACTIVATE_EMPTY);
> -		discard_slab(s, slab);
> -		stat(s, FREE_SLAB);
> -	} else if (mode == M_FULL_NOLIST) {
> +	} else
>  		stat(s, DEACTIVATE_FULL);
> -	}

Same here.

Thanks!

>  }
>  
>  #ifdef CONFIG_SLUB_CPU_PARTIAL