netdev - Re: [Intel-wired-lan] [net-next PATCH v3 1/3] e1000: track BQL bytes regardless of skb or not

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Date:   Tue, 13 Sep 2016 06:17:35 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        William Tu <u9012063@...il.com>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [Intel-wired-lan] [net-next PATCH v3 1/3] e1000: track BQL bytes
 regardless of skb or not

On Mon, Sep 12, 2016 at 9:25 PM, Tom Herbert <tom@...bertland.com> wrote:
> On Mon, Sep 12, 2016 at 8:00 PM, Alexander Duyck
> <alexander.duyck@...il.com> wrote:
>> On Mon, Sep 12, 2016 at 3:13 PM, John Fastabend
>> <john.fastabend@...il.com> wrote:
>>> The BQL API does not reference the sk_buff nor does the driver need to
>>> reference the sk_buff to calculate the length of a transmitted frame.
>>> This patch removes an sk_buff reference from the xmit irq path and
>>> also allows packets sent from XDP to use BQL.
>>>
>>> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
>>> ---
>>>  drivers/net/ethernet/intel/e1000/e1000_main.c |    7 ++-----
>>>  1 file changed, 2 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c b/drivers/net/ethernet/intel/e1000/e1000_main.c
>>> index f42129d..62a7f8d 100644
>>> --- a/drivers/net/ethernet/intel/e1000/e1000_main.c
>>> +++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
>>> @@ -3882,11 +3882,8 @@ static bool e1000_clean_tx_irq(struct e1000_adapter *adapter,
>>>                         if (cleaned) {
>>>                                 total_tx_packets += buffer_info->segs;
>>>                                 total_tx_bytes += buffer_info->bytecount;
>>> -                               if (buffer_info->skb) {
>>> -                                       bytes_compl += buffer_info->skb->len;
>>> -                                       pkts_compl++;
>>> -                               }
>>> -
>>> +                               bytes_compl += buffer_info->length;
>>> +                               pkts_compl++;
>>>                         }
>>>                         e1000_unmap_and_free_tx_resource(adapter, buffer_info);
>>>                         tx_desc->upper.data = 0;
>>
>> Actually it might be worth looking into why we have two different
>> stats for tracking bytecount and segs.  From what I can tell the
>> pkts_compl value is never actually used.  The function doesn't even
>> use the value so it is just wasted cycles.  And as far as the bytes go
>
> Transmit flow steering which I posted and is pending on some testing
> uses the pkt count BQL to track inflight packets.
>
>> the accounting would be more accurate if you were to use bytecount
>> instead of buffer_info->skb->len.  You would just need to update the
>> xmit function to use that on the other side so that they match.

Okay, that makes sense.

But as I was saying we might be better off using the segs and
bytecount values instead of the skb->len in the xmit and cleanup paths
to get more accurate accounting for the total bytes/packets coming and
going from the interface.  That way we can avoid any significant
change in behavior between TSO and GSO.

- Alex