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

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Date:   Tue, 18 Jun 2019 12:00:12 +0000
From:   Maxim Mikityanskiy <maximmi@...lanox.com>
To:     Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
CC:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        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>
Subject: Re: [PATCH bpf-next v4 07/17] libbpf: Support drivers with
 non-combined channels

On 2019-06-14 20:15, Maciej Fijalkowski wrote:
> On Fri, 14 Jun 2019 13:25:24 +0000
> Maxim Mikityanskiy <maximmi@...lanox.com> wrote:
> 
>> On 2019-06-13 17:45, Maciej Fijalkowski wrote:
>>> On Thu, 13 Jun 2019 14:01:39 +0000
>>> Maxim Mikityanskiy <maximmi@...lanox.com> 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 <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);
>>>
>>> So in case of 32 HW queues you'd like to get 64 entries in xskmap?
>>
>> "32 HW queues" is not quite correct. It will be 32 combined channels,
>> each with one regular RX queue and one XSK RX queue (regular RX queues
>> are part of RSS). In this case, I'll have 64 XSKMAP entries.
>>
>>> Do you still
>>> have a need for attaching the xsksocks to the RSS queues?
>>
>> You can attach an XSK to a regular RX queue, but not in zero-copy mode.
>> The intended use is, of course, to attach XSKs to XSK RX queues in
>> zero-copy mode.
>>
>>> 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?
>>
>> This is correct, but these entries are still needed if one decides to
>> run compatibility mode without zero-copy on queues 0..31.
> 
> Why would I want to run AF_XDP without ZC? The main reason for having AF_XDP
> support in drivers is the zero copy, right?

Yes, AF_XDP is intended to be used with zero copy when the driver 
implements it. I'm not breaking the compatibility mode if I can keep it 
supported.

> Besides that, are you educating the user in some way which queue ids should be
> used so there's ZC in picture? If that was already asked/answered, then sorry
> about that.

The details about queue IDs are in the commit message for the final patch.

>>
>>>    
>>>>>> +
>>>>>> +	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);
>>>>>       
>>>>   
>>>    
>>
>