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

On Wed, 1 Nov 2017 06:54:46 -0700
John Fastabend [off-list ref] wrote:

On 11/01/2017 04:44 AM, Jesper Dangaard Brouer wrote:

quoted

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 <redacted>
---
 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?

I calculate this into an approx 2 nanosec improvement based on PPS
benchmarks.  Given my systems accuracy is around 2 nanosec (after much
tuning) then I cannot claim my measurements to be statistically
significant ;-)

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

Thanks

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer