Re: [PATCH 2/2] mm, page_alloc: use unlikely() in task_capc()

[Hugh Dickins]

On Tue, 16 Jun 2020, Vlastimil Babka wrote:

> Hugh noted that task_capc() could use unlikely(), as most of the time there is
> no capture in progress and we are in page freeing hot path. Indeed adding
> unlikely() redirects produces assembly that better matches the assumption and
> moves all the tests away from the hot path.
> 
> I have also noticed that we don't need to test for cc->direct_compaction as the
> only place we set current->task_capture is compact_zone_order() which also
> always sets cc->direct_compaction true.
> 
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Hugh Dickins <hughd@googlecom>

Thanks for pursuing these, Vlastimil: I'm glad you were able
to remove a test and branch instead of adding one as I had.

One little thing, you've probably gone into this yourself and know
what you've written here is optimal: but I'd rather imagined it with
"unlikely(capc) && ..." instead of "unlikely(capc && ...)" - no need
to respond, please just give it a moment's consideration, Acked anyway.

> ---
>  mm/page_alloc.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48eb0f1410d4..8a4e342d7e8f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -813,11 +813,10 @@ static inline struct capture_control *task_capc(struct zone *zone)
>  {
>  	struct capture_control *capc = current->capture_control;
>  
> -	return capc &&
> +	return unlikely(capc &&
>  		!(current->flags & PF_KTHREAD) &&
>  		!capc->page &&
> -		capc->cc->zone == zone &&
> -		capc->cc->direct_compaction ? capc : NULL;
> +		capc->cc->zone == zone) ? capc : NULL;
>  }
>  
>  static inline bool
> -- 
> 2.27.0
> 
>