From bd9e89f29427f4accd62ce5dfe1547549f0d3fbc Mon Sep 17 00:00:00 2001 From: Claudiu Manoil Date: Thu, 14 Feb 2013 05:00:04 +0000 Subject: [PATCH] gianfar: GRO_DROP is unlikely The change is significant since it affects the rx hot path. Paul observed and documented the effects at asm level, see below: "It turns out that it does make a difference, since gfar_process_frame gets inlined, and so the increment code gets moved out of line (I have marked the if statment with * and the increment code within "-----"): ------------------------- as is currently ------------------ 4d14: 80 61 00 18 lwz r3,24(r1) 4d18: 7f c4 f3 78 mr r4,r30 4d1c: 48 00 00 01 bl 4d1c * 4d20: 2f 83 00 04 cmpwi cr7,r3,4 4d24: 40 9e 00 1c bne- cr7,4d40 ---------------------------- 4d28: 81 3c 01 f8 lwz r9,504(r28) 4d2c: 81 5c 01 fc lwz r10,508(r28) 4d30: 31 4a 00 01 addic r10,r10,1 4d34: 7d 29 01 94 addze r9,r9 4d38: 91 3c 01 f8 stw r9,504(r28) 4d3c: 91 5c 01 fc stw r10,508(r28) ---------------------------- 4d40: a0 1f 00 24 lhz r0,36(r31) 4d44: 81 3f 00 00 lwz r9,0(r31) 4d48: 7f a4 eb 78 mr r4,r29 4d4c: 7f e3 fb 78 mr r3,r31 -------------------------- unlikely ------------------------ 4d14: 80 61 00 18 lwz r3,24(r1) 4d18: 7f c4 f3 78 mr r4,r30 4d1c: 48 00 00 01 bl 4d1c * 4d20: 2f 83 00 04 cmpwi cr7,r3,4 4d24: 41 9e 03 94 beq- cr7,50b8 4d28: a0 1f 00 24 lhz r0,36(r31) 4d2c: 81 3f 00 00 lwz r9,0(r31) 4d30: 7f a4 eb 78 mr r4,r29 4d34: 7f e3 fb 78 mr r3,r31 [...] 50b8: 81 3c 01 f8 lwz r9,504(r28) 50bc: 81 5c 01 fc lwz r10,508(r28) 50c0: 31 4a 00 01 addic r10,r10,1 50c4: 7d 29 01 94 addze r9,r9 50c8: 91 3c 01 f8 stw r9,504(r28) 50cc: 91 5c 01 fc stw r10,508(r28) 50d0: 4b ff fc 58 b 4d28 So, the increment does actually get moved ~1k away." Cc: Paul Gortmaker Signed-off-by: Claudiu Manoil Signed-off-by: David S. Miller --- drivers/net/ethernet/freescale/gianfar.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/net/ethernet/freescale/gianfar.c b/drivers/net/ethernet/freescale/gianfar.c index 592d297..af0f4d6 100644 --- a/drivers/net/ethernet/freescale/gianfar.c +++ b/drivers/net/ethernet/freescale/gianfar.c @@ -2740,7 +2740,7 @@ static int gfar_process_frame(struct net_device *dev, struct sk_buff *skb, /* Send the packet up the stack */ ret = napi_gro_receive(napi, skb); - if (GRO_DROP == ret) + if (unlikely(GRO_DROP == ret)) atomic64_inc(&priv->extra_stats.kernel_dropped); return 0; -- 2.7.4