Re: [PATCH 6/9] mm, slub: introduce static key for slub_debug()

[Roman Gushchin]

On Wed, Jun 10, 2020 at 06:31:32PM +0200, Vlastimil Babka wrote:
> One advantage of CONFIG_SLUB_DEBUG is that a generic distro kernel can be built
> with the option enabled, but it's inactive until simply enabled on boot,
> without rebuilding the kernel. With a static key, we can further eliminate the
> overhead of checking whether a cache has a particular debug flag enabled if we
> know that there are no such caches (slub_debug was not enabled during boot). We
> use the same mechanism also for e.g.  page_owner, debug_pagealloc or kmemcg
> functionality.
> 
> This patch introduces the static key and makes the general check for per-cache
> debug flags kmem_cache_debug() use it. This benefits several call sites,
> including (slow path but still rather frequent) __slab_free(). The next patches
> will add more uses.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Roman Gushchin <guro@xxxxxx>

Thanks!

> ---
>  mm/slub.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 8dd2925448ae..24d3e5f832aa 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -114,13 +114,21 @@
>   * 			the fast path and disables lockless freelists.
>   */
>  
> +#ifdef CONFIG_SLUB_DEBUG
> +#ifdef CONFIG_SLUB_DEBUG_ON
> +DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
> +#else
> +DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
> +#endif
> +#endif
> +
>  static inline int kmem_cache_debug(struct kmem_cache *s)
>  {
>  #ifdef CONFIG_SLUB_DEBUG
> -	return unlikely(s->flags & SLAB_DEBUG_FLAGS);
> -#else
> -	return 0;
> +	if (static_branch_unlikely(&slub_debug_enabled))
> +		return s->flags & SLAB_DEBUG_FLAGS;
>  #endif
> +	return 0;
>  }
>  
>  void *fixup_red_left(struct kmem_cache *s, void *p)
> @@ -1389,6 +1397,8 @@ static int __init setup_slub_debug(char *str)
>  		slub_debug_string = saved_str;
>  	}
>  out:
> +	if (slub_debug != 0 || slub_debug_string)
> +		static_branch_enable(&slub_debug_enabled);
>  	if ((static_branch_unlikely(&init_on_alloc) ||
>  	     static_branch_unlikely(&init_on_free)) &&
>  	    (slub_debug & SLAB_POISON))
> -- 
> 2.26.2
>