Re: [PATCH 07/11] mm/slab: racy access/modify the slab color

[Christoph Lameter]

On Mon, 28 Mar 2016, js1304@xxxxxxxxx wrote:

> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>
> Slab color isn't needed to be changed strictly. Because locking
> for changing slab color could cause more lock contention so this patch
> implements racy access/modify the slab color. This is a preparation step
> to implement lockless allocation path when there is no free objects in
> the kmem_cache.

Acked-by: Christoph Lameter <cl@xxxxxxxxx>

The rest of the description does not relate to this patch and does not
actually reflect the improvement of applying this patch. Remove the rest?


> Below is the result of concurrent allocation/free in slab allocation
> benchmark made by Christoph a long time ago. I make the output simpler.
> The number shows cycle count during alloc/free respectively so less
> is better.
>
> * Before
> Kmalloc N*alloc N*free(32): Average=365/806
> Kmalloc N*alloc N*free(64): Average=452/690
> Kmalloc N*alloc N*free(128): Average=736/886
> Kmalloc N*alloc N*free(256): Average=1167/985
> Kmalloc N*alloc N*free(512): Average=2088/1125
> Kmalloc N*alloc N*free(1024): Average=4115/1184
> Kmalloc N*alloc N*free(2048): Average=8451/1748
> Kmalloc N*alloc N*free(4096): Average=16024/2048
>
> * After
> Kmalloc N*alloc N*free(32): Average=355/750
> Kmalloc N*alloc N*free(64): Average=452/812
> Kmalloc N*alloc N*free(128): Average=559/1070
> Kmalloc N*alloc N*free(256): Average=1176/980
> Kmalloc N*alloc N*free(512): Average=1939/1189
> Kmalloc N*alloc N*free(1024): Average=3521/1278
> Kmalloc N*alloc N*free(2048): Average=7152/1838
> Kmalloc N*alloc N*free(4096): Average=13438/2013
>
> It shows that contention is reduced for object size >= 1024
> and performance increases by roughly 15%.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>  mm/slab.c | 26 +++++++++++++-------------
>  1 file changed, 13 insertions(+), 13 deletions(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index df11757..52fc5e3 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2536,20 +2536,7 @@ static int cache_grow(struct kmem_cache *cachep,
>  	}
>  	local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);
>
> -	/* Take the node list lock to change the colour_next on this node */
>  	check_irq_off();
> -	n = get_node(cachep, nodeid);
> -	spin_lock(&n->list_lock);
> -
> -	/* Get colour for the slab, and cal the next value. */
> -	offset = n->colour_next;
> -	n->colour_next++;
> -	if (n->colour_next >= cachep->colour)
> -		n->colour_next = 0;
> -	spin_unlock(&n->list_lock);
> -
> -	offset *= cachep->colour_off;
> -
>  	if (gfpflags_allow_blocking(local_flags))
>  		local_irq_enable();
>
> @@ -2570,6 +2557,19 @@ static int cache_grow(struct kmem_cache *cachep,
>  	if (!page)
>  		goto failed;
>
> +	n = get_node(cachep, nodeid);
> +
> +	/* Get colour for the slab, and cal the next value. */
> +	n->colour_next++;
> +	if (n->colour_next >= cachep->colour)
> +		n->colour_next = 0;
> +
> +	offset = n->colour_next;
> +	if (offset >= cachep->colour)
> +		offset = 0;
> +
> +	offset *= cachep->colour_off;
> +
>  	/* Get slab management. */
>  	freelist = alloc_slabmgmt(cachep, page, offset,
>  			local_flags & ~GFP_CONSTRAINT_MASK, nodeid);
>