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

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Date:   Thu, 13 Jun 2019 14:01:39 +0000
From:   Maxim Mikityanskiy <maximmi@...lanox.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jonathan Lemon <bsd@...com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
Subject: Re: [PATCH bpf-next v4 07/17] libbpf: Support drivers with
 non-combined channels

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