Re: [net-next PATCH] bpf: cpumap micro-optimization in cpu_map_enqueue

[John Fastabend <john.fastabend@gmail.com>]

On 11/01/2017 04:44 AM, Jesper Dangaard Brouer wrote:
> Discovered that the compiler laid-out asm code in suboptimal way
> when studying perf report during benchmarking of cpumap. Help
> the compiler by the marking unlikely code paths.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  kernel/bpf/cpumap.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index 86e29cbf7827..ce5b669003b2 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -208,7 +208,7 @@ static struct xdp_pkt *convert_to_xdp_pkt(struct xdp_buff *xdp)
>  	headroom = xdp->data - xdp->data_hard_start;
>  	metasize = xdp->data - xdp->data_meta;
>  	metasize = metasize > 0 ? metasize : 0;
> -	if ((headroom - metasize) < sizeof(*xdp_pkt))
> +	if (unlikely((headroom - metasize) < sizeof(*xdp_pkt)))
>  		return NULL;
>  
>  	/* Store info in top of packet */
> @@ -656,7 +656,7 @@ int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp,
>  	struct xdp_pkt *xdp_pkt;
>  
>  	xdp_pkt = convert_to_xdp_pkt(xdp);
> -	if (!xdp_pkt)
> +	if (unlikely(!xdp_pkt))
>  		return -EOVERFLOW;
>  
>  	/* Info needed when constructing SKB on remote CPU */
> 

Seems OK to me, just curious is this noticeable at pps benchmarks?

Acked-by: John Fastabend <john.fastabend@gmail.com>