Eric Dumazet <eric.dumazet@xxxxxxxxx> writes:
> 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 ?
>
Simon, could you please test this patch for IPv6 and show us the numbers?
> 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.
>
My guess is that this is not the bottleneck, something is happening
behind the scene with out packets in Hyper-V host (e.g. re-distributing
them to hardware queues?) but I don't know the internals, Microsoft
folks could probably comment.
> 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.
>
Alternative patch which uses skb_get_hash() attached. Simon, could you
please share the rest (environment, metodology, numbers) with us here?
Thanks!
> Thanks.
>
> PS: For the time being this patch can probably be applied on -net tree,
> as it fixes a real bug.
--
Vitaly
>From 0040e79c1303bd225ddbbce679ea944ea11ad0bd Mon Sep 17 00:00:00 2001
From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Date: Wed, 6 Jan 2016 12:14:10 +0100
Subject: [PATCH] hv_netvsc: use skb_get_hash() instead of a homegrown
implementation
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
drivers/net/hyperv/netvsc_drv.c | 67 ++---------------------------------------
1 file changed, 3 insertions(+), 64 deletions(-)
diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
index 409b48e..038bf4f 100644
--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c
@@ -195,65 +195,6 @@ static void *init_ppi_data(struct rndis_message *msg, u32 ppi_size,
return ppi;
}
-union sub_key {
- u64 k;
- struct {
- u8 pad[3];
- u8 kb;
- u32 ka;
- };
-};
-
-/* Toeplitz hash function
- * data: network byte order
- * return: host byte order
- */
-static u32 comp_hash(u8 *key, int klen, void *data, int dlen)
-{
- union sub_key subk;
- int k_next = 4;
- u8 dt;
- int i, j;
- u32 ret = 0;
-
- subk.k = 0;
- subk.ka = ntohl(*(u32 *)key);
-
- for (i = 0; i < dlen; i++) {
- subk.kb = key[k_next];
- k_next = (k_next + 1) % klen;
- dt = ((u8 *)data)[i];
- for (j = 0; j < 8; j++) {
- if (dt & 0x80)
- ret ^= subk.ka;
- dt <<= 1;
- subk.k <<= 1;
- }
- }
-
- return ret;
-}
-
-static bool netvsc_set_hash(u32 *hash, struct sk_buff *skb)
-{
- struct flow_keys flow;
- int data_len;
-
- if (!skb_flow_dissect_flow_keys(skb, &flow, 0) ||
- !(flow.basic.n_proto == htons(ETH_P_IP) ||
- flow.basic.n_proto == htons(ETH_P_IPV6)))
- return false;
-
- if (flow.basic.ip_proto == IPPROTO_TCP)
- data_len = 12;
- else
- data_len = 8;
-
- *hash = comp_hash(netvsc_hash_key, HASH_KEYLEN, &flow, data_len);
-
- return true;
-}
-
static u16 netvsc_select_queue(struct net_device *ndev, struct sk_buff *skb,
void *accel_priv, select_queue_fallback_t fallback)
{
@@ -266,11 +207,9 @@ static u16 netvsc_select_queue(struct net_device *ndev, struct sk_buff *skb,
if (nvsc_dev == NULL || ndev->real_num_tx_queues <= 1)
return 0;
- if (netvsc_set_hash(&hash, skb)) {
- q_idx = nvsc_dev->send_table[hash % VRSS_SEND_TAB_SIZE] %
- ndev->real_num_tx_queues;
- skb_set_hash(skb, hash, PKT_HASH_TYPE_L3);
- }
+ hash = skb_get_hash(skb);
+ q_idx = nvsc_dev->send_table[hash % VRSS_SEND_TAB_SIZE] %
+ ndev->real_num_tx_queues;
return q_idx;
}
--
2.4.3
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