Re: [PATCH net-next] hv_netvsc: don't make assumptions on struct flow_keys layout

On 16-01-07 07:49 PM, KY Srinivasan wrote:

quoted

-----Original Message-----
From: John Fastabend [mailto:john.fastabend@gmail.com]
Sent: Thursday, January 7, 2016 5:02 PM
To: Vitaly Kuznetsov <vkuznets@redhat.com>; Simon Xiao
[off-list ref]; Eric Dumazet [off-list ref]
Cc: Tom Herbert <redacted>; netdev@vger.kernel.org; KY
Srinivasan [off-list ref]; Haiyang Zhang [off-list ref];
devel@linuxdriverproject.org; linux-kernel@vger.kernel.org; David Miller
[off-list ref]
Subject: Re: [PATCH net-next] hv_netvsc: don't make assumptions on struct
flow_keys layout

On 16-01-07 05:28 AM, Vitaly Kuznetsov wrote:

quoted

Eric Dumazet [off-list ref] writes:

quoted

On Thu, 2016-01-07 at 10:33 +0100, Vitaly Kuznetsov wrote:

quoted

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.

Also can I ask the maybe naive question. It looks like the hypervisor
is populating some table via a mailbox msg and this is used to select
the queues I guess with some sort of weighting function?

What happens if you just remove select_queue altogether? Or maybe just
what is this 16 entry table doing? How does this work on my larger
systems with 64+ cores can I only use 16 cores? Sorry I really have
no experience with hyperV and this got me curious.

We will limit the number of VRSS channels to the number of CPUs in
a NUMA node. If the number of CPUs in a NUMA node exceeds 8, we
will only open up 8 VRSS channels. On the host side currently traffic
spreading is done in software and we have found that limiting to 8 CPUs
gives us the best throughput. In Windows Server 2016, we will be 
distributing traffic on the host in hardware; the heuristics in the guest
may change.

Regards,

K. Y

I think a better way to do this would be to query the numa node when
the interface comes online via dev_to_node() and then use cpu_to_node()
or create/find some better variant to get a list of cpus on the numa
node.

At this point you can use the xps mapping interface
netif_set_xps_queue() to get the right queue to cpu binding. If you want
to cap it to max 8 queues that works as well. I don't think there is
any value to have more tx queues than number of cpus in use.

If you do it this way all the normal mechanisms to setup queue mappings
will work for users who are doing some special configuration and the
default will still be what you want.

I guess I should go do this numa mapping for ixgbe and friends now that
I mention it. Last perf numbers I had showed cross numa affinitizing
was pretty bad.

Thanks,
John