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Date:   Wed, 25 Apr 2018 17:04:49 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Christopher Lameter <cl@linux.com>
cc:     Mike Snitzer <snitzer@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        Pekka Enberg <penberg@kernel.org>, linux-mm@kvack.org,
        dm-devel@redhat.com, David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE
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On Wed, 18 Apr 2018, Christopher Lameter wrote:

> On Tue, 17 Apr 2018, Mikulas Patocka wrote:
> 
> > I can make a slub-only patch with no extra flag (on a freshly booted
> > system it increases only the order of caches "TCPv6" and "sighand_cache"
> > by one - so it should not have unexpected effects):
> >
> > Doing a generic solution for slab would be more comlpicated because slab
> > assumes that all slabs have the same order, so it can't fall-back to
> > lower-order allocations.
> 
> Well again SLAB uses compound pages and thus would be able to detect the
> size of the page. It may be some work but it could be done.
> 
> >
> > Index: linux-2.6/mm/slub.c
> > ===================================================================
> > --- linux-2.6.orig/mm/slub.c	2018-04-17 19:59:49.000000000 +0200
> > +++ linux-2.6/mm/slub.c	2018-04-17 20:58:23.000000000 +0200
> > @@ -3252,6 +3252,7 @@ static inline unsigned int slab_order(un
> >  static inline int calculate_order(unsigned int size, unsigned int reserved)
> >  {
> >  	unsigned int order;
> > +	unsigned int test_order;
> >  	unsigned int min_objects;
> >  	unsigned int max_objects;
> >
> > @@ -3277,7 +3278,7 @@ static inline int calculate_order(unsign
> >  			order = slab_order(size, min_objects,
> >  					slub_max_order, fraction, reserved);
> >  			if (order <= slub_max_order)
> > -				return order;
> > +				goto ret_order;
> >  			fraction /= 2;
> >  		}
> >  		min_objects--;
> > @@ -3289,15 +3290,25 @@ static inline int calculate_order(unsign
> >  	 */
> >  	order = slab_order(size, 1, slub_max_order, 1, reserved);
> 
> The slab order is determined in slab_order()
> 
> >  	if (order <= slub_max_order)
> > -		return order;
> > +		goto ret_order;
> >
> >  	/*
> >  	 * Doh this slab cannot be placed using slub_max_order.
> >  	 */
> >  	order = slab_order(size, 1, MAX_ORDER, 1, reserved);
> > -	if (order < MAX_ORDER)
> > -		return order;
> > -	return -ENOSYS;
> > +	if (order >= MAX_ORDER)
> > +		return -ENOSYS;
> > +
> > +ret_order:
> > +	for (test_order = order + 1; test_order < MAX_ORDER; test_order++) {
> > +		unsigned long order_objects = ((PAGE_SIZE << order) - reserved) / size;
> > +		unsigned long test_order_objects = ((PAGE_SIZE << test_order) - reserved) / size;
> > +		if (test_order_objects > min(32, MAX_OBJS_PER_PAGE))
> > +			break;
> > +		if (test_order_objects > order_objects << (test_order - order))
> > +			order = test_order;
> > +	}
> > +	return order;
> 
> Could yo move that logic into slab_order()? It does something awfully
> similar.

But slab_order (and its caller) limits the order to "max_order" and we 
want more.

Perhaps slab_order should be dropped and calculate_order totally 
rewritten?

Mikulas