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

[Maxim Mikityanskiy <maximmi@mellanox.com>]

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 <maximmi@mellanox.com>
>> Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
>> Acked-by: Saeed Mahameed <saeedm@mellanox.com>
> 
> 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);
>> +
>> +	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);
>