Re: [RFC 1/3] zsmalloc: move pages_allocated to zs_pool

[Seth Jennings <sjennings@variantweb.net>]

On Tue, Aug 05, 2014 at 05:02:01PM +0900, Minchan Kim wrote:
> Pages_allocated has counted in size_class structure and when user
> want to see total_size_bytes, it gathers all of value from each
> size_class to report the sum.
> 
> It's not bad if user don't see the value often but if user start
> to see the value frequently, it would be not a good deal for
> performance POV.
> 
> This patch moves the variable from size_class to zs_pool so it would
> reduce memory footprint (from [255 * 8byte] to [sizeof(atomic_t)])
> but it adds new locking overhead but it wouldn't be severe because
> it's not a hot path in zs_malloc(ie, it is called only when new
> zspage is created, not a object).
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  mm/zsmalloc.c | 30 ++++++++++++++++--------------
>  1 file changed, 16 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index fe78189624cf..a6089bd26621 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -198,9 +198,6 @@ struct size_class {
>  
>  	spinlock_t lock;
>  
> -	/* stats */
> -	u64 pages_allocated;
> -
>  	struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];
>  };
>  
> @@ -216,9 +213,12 @@ struct link_free {
>  };
>  
>  struct zs_pool {
> +	spinlock_t stat_lock;
> +
>  	struct size_class size_class[ZS_SIZE_CLASSES];
>  
>  	gfp_t flags;	/* allocation flags used when growing pool */
> +	unsigned long pages_allocated;

As Dan was saying, I think this can be atomic to avoid the locking.

Seth

>  };
>  
>  /*
> @@ -882,6 +882,7 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>  
>  	}
>  
> +	spin_lock_init(&pool->stat_lock);
>  	pool->flags = flags;
>  
>  	return pool;
> @@ -943,8 +944,10 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>  			return 0;
>  
>  		set_zspage_mapping(first_page, class->index, ZS_EMPTY);
> +		spin_lock(&pool->stat_lock);
> +		pool->pages_allocated += class->pages_per_zspage;
> +		spin_unlock(&pool->stat_lock);
>  		spin_lock(&class->lock);
> -		class->pages_allocated += class->pages_per_zspage;
>  	}
>  
>  	obj = (unsigned long)first_page->freelist;
> @@ -997,14 +1000,14 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
>  
>  	first_page->inuse--;
>  	fullness = fix_fullness_group(pool, first_page);
> -
> -	if (fullness == ZS_EMPTY)
> -		class->pages_allocated -= class->pages_per_zspage;
> -
>  	spin_unlock(&class->lock);
>  
> -	if (fullness == ZS_EMPTY)
> +	if (fullness == ZS_EMPTY) {
> +		spin_lock(&pool->stat_lock);
> +		pool->pages_allocated -= class->pages_per_zspage;
> +		spin_unlock(&pool->stat_lock);
>  		free_zspage(first_page);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(zs_free);
>  
> @@ -1100,12 +1103,11 @@ EXPORT_SYMBOL_GPL(zs_unmap_object);
>  
>  u64 zs_get_total_size_bytes(struct zs_pool *pool)
>  {
> -	int i;
> -	u64 npages = 0;
> -
> -	for (i = 0; i < ZS_SIZE_CLASSES; i++)
> -		npages += pool->size_class[i].pages_allocated;
> +	u64 npages;
>  
> +	spin_lock(&pool->stat_lock);
> +	npages = pool->pages_allocated;
> +	spin_unlock(&pool->stat_lock);
>  	return npages << PAGE_SHIFT;
>  }
>  EXPORT_SYMBOL_GPL(zs_get_total_size_bytes);
> -- 
> 2.0.0
> 
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