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

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Date:   Tue, 8 Mar 2022 03:58:37 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, 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 Mon, Mar 07, 2022 at 05:40:42PM +0100, Vlastimil Babka wrote:
> 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.
>

Ah, you are right. Agree with this change.
"Remove unnecessary brace" rule does not apply here.

> > -		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).

Yes.

> 
> > -		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.
>

Yes.

> >  }
> >  
> >  #ifdef CONFIG_SLUB_CPU_PARTIAL
> 

Thanks!

-- 
Thank you, You are awesome!
Hyeonggon :-)