Subject: Re: [PATCH bpf-next v4 07/17] libbpf: Support drivers with non-combined channels

From: Maciej Fijalkowski <[email-masked]>
Date: Thu, 13 Jun 2019 16:45:37 +0200

On Thu, 13 Jun 2019 14:01:39 +0000
Maxim Mikityanskiy <[email-masked]> wrote:

> On 2019-06-12 23:23, Jakub Kicinski wrote:
> > On Wed, 12 Jun 2019 15:56:48 +0000, Maxim Mikityanskiy wrote:  
> >> Currently, libbpf uses the number of combined channels as the maximum
> >> queue number. However, the kernel has a different limitation:
> >>
> >> - xdp_reg_umem_at_qid() allows up to max(RX queues, TX queues).
> >>
> >> - ethtool_set_channels() checks for UMEMs in queues up to
> >>    combined_count + max(rx_count, tx_count).
> >>
> >> libbpf shouldn't limit applications to a lower max queue number. Account
> >> for non-combined RX and TX channels when calculating the max queue
> >> number. Use the same formula that is used in ethtool.
> >>
> >> Signed-off-by: Maxim Mikityanskiy <[email-masked]>
> >> Reviewed-by: Tariq Toukan <[email-masked]>
> >> Acked-by: Saeed Mahameed <[email-masked]>  
> > 
> > I don't think this is correct.  max_tx tells you how many TX channels
> > there can be, you can't add that to combined.  Correct calculations is:
> > 
> > max_num_chans = max(max_combined, max(max_rx, max_tx))  
> 
> First of all, I'm aligning with the formula in the kernel, which is:
> 
>      curr.combined_count + max(curr.rx_count, curr.tx_count);
>
> (see net/core/ethtool.c, ethtool_set_channels()).
> 
> The formula in libbpf should match it.
> 
> Second, the existing drivers have either combined channels or separate 
> rx and tx channels. So, for the first kind of drivers, max_tx doesn't 
> tell how many TX channels there can be, it just says 0, and max_combined 
> tells how many TX and RX channels are supported. As max_tx doesn't 
> include max_combined (and vice versa), we should add them up.
> 
> >>   tools/lib/bpf/xsk.c | 6 +++---
> >>   1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> >> index bf15a80a37c2..86107857e1f0 100644
> >> --- a/tools/lib/bpf/xsk.c
> >> +++ b/tools/lib/bpf/xsk.c
> >> @@ -334,13 +334,13 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
> >>   		goto out;
> >>   	}
> >>   
> >> -	if (channels.max_combined == 0 || errno == EOPNOTSUPP)
> >> +	ret = channels.max_combined + max(channels.max_rx, channels.max_tx);

So in case of 32 HW queues you'd like to get 64 entries in xskmap? Do you still
have a need for attaching the xsksocks to the RSS queues? I thought you want
them to be separated. So if I'm reading this right, [0, 31] xskmap entries
would be unused for the most of the time, no?

> >> +
> >> +	if (ret == 0 || errno == EOPNOTSUPP)
> >>   		/* If the device says it has no channels, then all traffic
> >>   		 * is sent to a single stream, so max queues = 1.
> >>   		 */
> >>   		ret = 1;
> >> -	else
> >> -		ret = channels.max_combined;
> >>   
> >>   out:
> >>   	close(fd);  
> >   
>