linux-kernel - Re: [PATCH v3 2/2] mm/slub: refactor deactivate

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Date:   Mon, 7 Mar 2022 17:40:42 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>,
        Matthew WilCox <willy@...radead.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] mm/slub: refactor deactivate_slab()

On 3/7/22 08:40, Hyeonggon Yoo wrote:
> Simplify deactivate_slab() by unlocking n->list_lock and retrying
> cmpxchg_double() when cmpxchg_double() fails, and perform
> add_{partial,full} only when it succeed.
> 
> Releasing and taking n->list_lock again here is not harmful as SLUB
> avoids deactivating slabs as much as possible.
> 
> [ vbabka@...e.cz: perform add_{partial,full} when cmpxchg_double()
>   succeed.
> 
>   count deactivating full slabs even if debugging flag is not set. ]
> 
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@...il.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

adding both to slab-next. Fixed up some nits myself, see below:

>  
> @@ -2420,61 +2416,50 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>  	new.frozen = 0;
>  
>  	if (!new.inuse && n->nr_partial >= s->min_partial)
> -		m = M_FREE;
> +		mode = M_FREE;
>  	else if (new.freelist) {

This was against kernel style even before the patch - we use { } in the
'else if' branch, thus all branches should use { } even if one-line.

> -		m = M_PARTIAL;
> -		if (!lock) {
> -			lock = 1;
> -			/*
> -			 * Taking the spinlock removes the possibility that
> -			 * acquire_slab() will see a slab that is frozen
> -			 */
> -			spin_lock_irqsave(&n->list_lock, flags);
> -		}
> -	} else {
> -		m = M_FULL;
> -		if (kmem_cache_debug_flags(s, SLAB_STORE_USER) && !lock) {
> -			lock = 1;
> -			/*
> -			 * This also ensures that the scanning of full
> -			 * slabs from diagnostic functions will not see
> -			 * any frozen slabs.
> -			 */
> -			spin_lock_irqsave(&n->list_lock, flags);
> -		}
> -	}
> -
> -	if (l != m) {
> -		if (l == M_PARTIAL)
> -			remove_partial(n, slab);
> -		else if (l == M_FULL)
> -			remove_full(s, n, slab);
> +		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 if (kmem_cache_debug_flags(s, SLAB_STORE_USER)) {
> +		mode = M_FULL;
> +		/*
> +		 * This also ensures that the scanning of full
> +		 * slabs from diagnostic functions will not see
> +		 * any frozen slabs.
> +		 */
> +		spin_lock_irqsave(&n->list_lock, flags);
> +	} else
> +		mode = M_FULL_NOLIST;

Ditto here (this is new).

> -		if (m == M_PARTIAL)
> -			add_partial(n, slab, tail);
> -		else if (m == M_FULL)
> -			add_full(s, n, slab);
> -	}
>  
> -	l = m;
>  	if (!cmpxchg_double_slab(s, slab,
>  				old.freelist, old.counters,
>  				new.freelist, new.counters,
> -				"unfreezing slab"))
> +				"unfreezing slab")) {
> +		if (mode == M_PARTIAL || mode == M_FULL)
> +			spin_unlock_irqrestore(&n->list_lock, flags);
>  		goto redo;
> +	}
>  
> -	if (lock)
> -		spin_unlock_irqrestore(&n->list_lock, flags);
>  
> -	if (m == M_PARTIAL)
> +	if (mode == M_PARTIAL) {
> +		add_partial(n, slab, tail);
> +		spin_unlock_irqrestore(&n->list_lock, flags);
>  		stat(s, tail);
> -	else if (m == M_FULL)
> -		stat(s, DEACTIVATE_FULL);
> -	else if (m == M_FREE) {
> +	} else if (mode == M_FREE) {
>  		stat(s, DEACTIVATE_EMPTY);
>  		discard_slab(s, slab);
>  		stat(s, FREE_SLAB);
> -	}
> +	} else if (mode == M_FULL) {
> +		add_full(s, n, slab);
> +		spin_unlock_irqrestore(&n->list_lock, flags);
> +		stat(s, DEACTIVATE_FULL);
> +	} else if (mode == M_FULL_NOLIST)
> +		stat(s, DEACTIVATE_FULL);

And here.

>  }
>  
>  #ifdef CONFIG_SLUB_CPU_PARTIAL