max channels for mlx5

max channels for mlx5

[David Ahern]

Hi Saeed:

When I saw this commit last year:

commit 57c7fce14b1ad512a42abe33cb721a2ea3520d4b
Author: Fan Li <fanl@mellanox.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

Re: max channels for mlx5

[Saeed Mahameed]

On Sun, 2020-05-03 at 18:41 -0600, David Ahern wrote:
> Hi Saeed:
> 
> When I saw this commit last year:
> 
> commit 57c7fce14b1ad512a42abe33cb721a2ea3520d4b
> Author: Fan Li <fanl@mellanox.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 patch should help, unless you are limited by FW/system MSI-x .. 

what is the amount of msix avaiable for eth0 port ?

businfo=$(ethtool -i eth0 | grep bus-info | cut -d":" -f2-)
cat /proc/interrupts | grep $businfo | wc -l

> 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
> 

So if number of msix is 64, we can only use 63 for data path
completions .. 

do you have sriov enabled ? 

what is the FW version you have ?
we need to figure out if this is a system MSIX limitation or a FW
limitation.

> 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.
> 

Know XDP redirect issue,  you need to tune the RSS and affinity on RX
side and match TX count and affinity on TX side, so you won't end up on
a wrong CPU on the TX side
 
> Am I missing something about how to use the expanded maximum?
> 
> David

Re: max channels for mlx5

[David Ahern]

On 5/4/20 3:46 PM, Saeed Mahameed wrote:
> what is the amount of msix avaiable for eth0 port ?
> 
> businfo=$(ethtool -i eth0 | grep bus-info | cut -d":" -f2-)
> cat /proc/interrupts | grep $businfo | wc -l

64

> 
> So if number of msix is 64, we can only use 63 for data path
> completions .. 
> 
> do you have sriov enabled ? 

no

> 
> what is the FW version you have ?

$ ethtool -i eth0
driver: mlx5_core
version: 5.0-0
firmware-version: 14.27.1016 (MT_2420110034)

> we need to figure out if this is a system MSIX limitation or a FW
> limitation.
> 
>> 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.
>>
> 
> Know XDP redirect issue,  you need to tune the RSS and affinity on RX
> side and match TX count and affinity on TX side, so you won't end up on
> a wrong CPU on the TX side

Understood for port to port redirect.

This use case is a virtual machine with a tap device + vhost bypassing
host kernel stack and redirecting to a port. Losing a third of the cpus
for vhost threads is a huge limitation.