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Date: Thu, 07 Jan 2016 04:52:30 -0800 From: Eric Dumazet <eric.dumazet@...il.com> To: Vitaly Kuznetsov <vkuznets@...hat.com>, Tom Herbert <tom@...bertland.com> Cc: netdev@...r.kernel.org, "K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, devel@...uxdriverproject.org, linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net> Subject: Re: [PATCH net-next] hv_netvsc: don't make assumptions on struct flow_keys layout On Thu, 2016-01-07 at 10:33 +0100, Vitaly Kuznetsov wrote: > Recent changes to 'struct flow_keys' (e.g commit d34af823ff40 ("net: Add > VLAN ID to flow_keys")) introduced a performance regression in netvsc > driver. Is problem is, however, not the above mentioned commit but the > fact that netvsc_set_hash() function did some assumptions on the struct > flow_keys data layout and this is wrong. We need to extract the data we > need (src/dst addresses and ports) after the dissect. > > The issue could also be solved in a completely different way: as suggested > by Eric instead of our own homegrown netvsc_set_hash() we could use > skb_get_hash() which does more or less the same. Unfortunately, the > testing done by Simon showed that Hyper-V hosts are not happy with our > Jenkins hash, selecting the output queue with the current algorithm based > on Toeplitz hash works significantly better. Were tests done on IPv6 traffic ? Toeplitz hash takes at least 100 ns to hash 12 bytes (one iteration per bit : 96 iterations) For IPv6 it is 3 times this, since we have to hash 36 bytes. I do not see how it can compete with skb_get_hash() that directly gives skb->hash for local TCP flows. See commits b73c3d0e4f0e1961e15bec18720e48aabebe2109 ("net: Save TX flow hash in sock and set in skbuf on xmit") and 877d1f6291f8e391237e324be58479a3e3a7407c ("net: Set sk_txhash from a random number") I understand Microsoft loves Toeplitz, but this looks not well placed here. I suspect there is another problem. Please share your numbers and test methodology, and the alternative patch Simon tested so that we can double check it. Thanks. PS: For the time being this patch can probably be applied on -net tree, as it fixes a real bug. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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