Re: [RFC PATCH] mm: Micro-optimisation: Save two branches on hot - page allocation path

[Vlastimil Babka]

Let's CC Mel.

On 3/4/20 3:22 PM, mateusznosek0@xxxxxxxxx wrote:
> From: Mateusz Nosek <mateusznosek0@xxxxxxxxx>
> 
> This patch makes ALLOC_KSWAPD
> equal to __GFP_KSWAPD_RACLAIM (cast to 'int').
> 
> Thanks to that code like:
> ```if (gfp_mask & __GFP_KSWAPD_RECLAIM)
> 		alloc_flags |= ALLOC_KSWAPD;```
> can be changed to:
> ```alloc_flags |= (__force int) (gfp_mask &__GFP_KSWAPD_RECLAIM);```
> Thanks to this one branch less is generated in the assembly.
> 
> In case of ALLOC_KSWAPD flag two branches are saved,
> first one in code that always executes in the beggining of page allocation
> and the second one in loop in page allocator slowpath.
> 
> Signed-off-by: Mateusz Nosek <mateusznosek0@xxxxxxxxx>

I think it's fine and in line with similar optimisations done by Mel in the past.

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Some nits below.

> ---
>  mm/internal.h   |  2 +-
>  mm/page_alloc.c | 23 +++++++++++++++--------
>  2 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 86372d164476..7fb724977743 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -535,7 +535,7 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>  #else
>  #define ALLOC_NOFRAGMENT	  0x0
>  #endif
> -#define ALLOC_KSWAPD		0x200 /* allow waking of kswapd */
> +#define ALLOC_KSWAPD		0x800 /* allow waking of kswapd */

Add a comment that the value has to match the GFP flag?

>  
>  enum ttu_flags;
>  struct tlbflush_unmap_batch;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 79e950d76ffc..73afd883eab5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3609,10 +3609,14 @@ static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
>  static inline unsigned int
>  alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
>  {
> -	unsigned int alloc_flags = 0;
> +	unsigned int alloc_flags;
>  
> -	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
> -		alloc_flags |= ALLOC_KSWAPD;
> +	/*
> +	 * __GFP_KSWAPD_RECLAIM is assumed to be the same as ALLOC_KSWAPD
> +	 * to save a branch.
> +	 */
> +	BUILD_BUG_ON(__GFP_KSWAPD_RECLAIM != (__force gfp_t) ALLOC_KSWAPD);

I think one BUILD_BUG_ON is enough...

> +	alloc_flags = (__force int) (gfp_mask & __GFP_KSWAPD_RECLAIM);
>  
>  #ifdef CONFIG_ZONE_DMA32
>  	if (!zone)
> @@ -4248,8 +4252,13 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  {
>  	unsigned int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
>  
> -	/* __GFP_HIGH is assumed to be the same as ALLOC_HIGH to save a branch. */
> +	/*
> +	 * __GFP_HIGH is assumed to be the same as ALLOC_HIGH
> +	 * and __GFP_KSWAPD_RECLAIM is assumed to be the same as ALLOC_KSWAPD
> +	 * to save two branches.
> +	 */
>  	BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
> +	BUILD_BUG_ON(__GFP_KSWAPD_RECLAIM != (__force gfp_t) ALLOC_KSWAPD);

... and this would be the better one of the two.

Thanks.

>  
>  	/*
>  	 * The caller may dip into page reserves a bit more if the caller
> @@ -4257,7 +4266,8 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
>  	 * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_HIGH (__GFP_HIGH).
>  	 */
> -	alloc_flags |= (__force int) (gfp_mask & __GFP_HIGH);
> +	alloc_flags |= (__force int)
> +		(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
>  
>  	if (gfp_mask & __GFP_ATOMIC) {
>  		/*
> @@ -4274,9 +4284,6 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	} else if (unlikely(rt_task(current)) && !in_interrupt())
>  		alloc_flags |= ALLOC_HARDER;
>  
> -	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
> -		alloc_flags |= ALLOC_KSWAPD;
> -
>  #ifdef CONFIG_CMA
>  	if (gfpflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
>  		alloc_flags |= ALLOC_CMA;
>