Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <07d4f687-544f-17d4-51cd-7b86aa23fb21@suse.cz>
Date:   Mon, 7 Mar 2022 17:40:42 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.6.1
Content-Language: en-US
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Marco Elver <elver@google.com>,
        Matthew WilCox <willy@infradead.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        linux-kernel@vger.kernel.org
References: <20220307074057.902222-1-42.hyeyoo@gmail.com>
 <20220307074057.902222-3-42.hyeyoo@gmail.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v3 2/2] mm/slub: refactor deactivate_slab()
In-Reply-To: <20220307074057.902222-3-42.hyeyoo@gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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@suse.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@gmail.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.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