netdev - max channels for mlx5

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Date:   Sun, 3 May 2020 18:41:40 -0600
From:   David Ahern <dsahern@...il.com>
To:     Saeed Mahameed <saeedm@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: max channels for mlx5

Hi Saeed:

When I saw this commit last year:

commit 57c7fce14b1ad512a42abe33cb721a2ea3520d4b
Author: Fan Li <fanl@...lanox.com>
Date:   Mon Dec 16 14:46:15 2019 +0200

    net/mlx5: Increase the max number of channels to 128

I was expecting to be able to increase the number of channels on larger
systems (e.g., 96 cpus), but that is not working as I expected.

This is on net-next as of today:
    60bcbc41ffb3 ("Merge branch 'net-smc-add-and-delete-link-processing'")

$ sudo ethtool -L eth0 combined 95
Cannot set device channel parameters: Invalid argument

As it stands the maximum is 63 (or is it 64 and cpus 0-63?):
$ sudo ethtool -l eth0
Channel parameters for eth0:
Pre-set maximums:
RX:		0
TX:		0
Other:		0
Combined:	63
Current hardware settings:
RX:		0
TX:		0
Other:		0
Combined:	63

A side effect of this limit is XDP_REDIRECT drops packets if a vhost
thread gets scheduled on cpus 64 and up since the tx queue is based on
processor id:

int mlx5e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
                   u32 flags)
{
	...
        sq_num = smp_processor_id();
        if (unlikely(sq_num >= priv->channels.num))
                return -ENXIO;

So in my example if the redirect happens on cpus 64-95, which is 1/3 of
my hardware threads, the packet is just dropped.

Am I missing something about how to use the expanded maximum?

David