[Intel-wired-lan] [RFC PATCH] ixgbe: delay tail write to every 'n' packets

[Alexander Duyck]

On Tue, Mar 14, 2017 at 4:50 PM, John Fastabend
<john.fastabend at gmail.com> wrote:
> On 17-03-13 10:18 AM, Alexander Duyck wrote:
>> On Mon, Mar 13, 2017 at 9:16 AM, John Fastabend
>> <john.fastabend at gmail.com> wrote:
>>> Current XDP implementation hits the tail on every XDP_TX return
>>> code. This patch changes driver behavior to only hit the tail after
>>> packet processing is complete.
>>>
>>> RFC for now as I test this, it looks promising on my dev box but
>>> want to do some more tests before official submission.
>>>
>>> Signed-off-by: John Fastabend <john.r.fastabend at intel.com>
>>> ---
>>>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |   14 +++++++++++---
>>>  1 file changed, 11 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
>>> index bef4e24..2c244b6 100644
>>> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
>>> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
>>> @@ -2282,6 +2282,7 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
>>>         unsigned int mss = 0;
>>>  #endif /* IXGBE_FCOE */
>>>         u16 cleaned_count = ixgbe_desc_unused(rx_ring);
>>> +       bool xdp_xmit = false;
>>>
>>>         while (likely(total_rx_packets < budget)) {
>>>                 union ixgbe_adv_rx_desc *rx_desc;
>>> @@ -2321,10 +2322,12 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
>>>                 }
>>>
>>>                 if (IS_ERR(skb)) {
>>> -                       if (PTR_ERR(skb) == -IXGBE_XDP_TX)
>>> +                       if (PTR_ERR(skb) == -IXGBE_XDP_TX) {
>>> +                               xdp_xmit = true;
>>>                                 ixgbe_rx_buffer_flip(rx_ring, rx_buffer, size);
>>> -                       else
>>> +                       } else {
>>>                                 rx_buffer->pagecnt_bias++;
>>> +                       }
>>>                         total_rx_packets++;
>>>                         total_rx_bytes += size;
>>>                 } else if (skb) {
>>> @@ -2392,6 +2395,12 @@ static int ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
>>>                 total_rx_packets++;
>>>         }
>>>
>>> +       if (xdp_xmit) {
>>> +               struct ixgbe_ring *ring = adapter->xdp_ring[smp_processor_id()];
>>> +
>>> +               writel(ring->next_to_use, ring->tail);
>>> +       }
>>> +
>>
>> We will need a wmb here.
>>
>>>         u64_stats_update_begin(&rx_ring->syncp);
>>>         rx_ring->stats.packets += total_rx_packets;
>>>         rx_ring->stats.bytes += total_rx_bytes;
>>> @@ -8251,7 +8260,6 @@ static int ixgbe_xmit_xdp_ring(struct ixgbe_adapter *adapter,
>>>         tx_buffer->next_to_watch = tx_desc;
>>>         ring->next_to_use = i;
>>>
>>> -       writel(i, ring->tail);
>>
>> So you might want to change the barrier setup for all this to use
>> smp_wmb instead.  We need the wmb to be paired with the writel.  That
>> should give you a slight performance boost since smp_wmb breaks down
>> to just a barrier on x86 systems.
>>
>
> Not sure I grok this description entirely, but I think you are just saying
> replace,
>
>         if (xdp_xmit) {
>                 ...
>                 writel(...)
>         }
>
> with
>
>         if (xdp_xmit) {
>                 ...
>                 smp_rmb()
>                 writel()
>         }
>
> Correct? Did you have some other change in mind ... "for all this"?
>
> Thanks,
> John


No.  Basically what it should be is find and replace wmb with
smp_wmb() in ixgbe_xmit_xdp_ring.  That way you won't have to worry
about a race between Tx and clean-up.

Then the if statement should be:
if (xdp_xmit) {
    ...
    /* wmb required to flush writes to coherent memory before writing
to non-coherent memory*/
    wmb();
    /* writel is a write to non-coherent memory mapped I/O */
    writel();
}

>>>         return IXGBE_XDP_TX;
>>>  }
>>>
>>>
>