netdev - Re: [PATCH net-next v2 2/5] virtio-net: transmit napi

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Date:   Fri, 21 Apr 2017 14:10:06 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v2 2/5] virtio-net: transmit napi

On Thu, Apr 20, 2017 at 12:02 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>>>   static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>>>   {
>>>         struct virtio_net_hdr_mrg_rxbuf *hdr;
>>> @@ -1130,9 +1172,11 @@ static netdev_tx_t start_xmit(struct sk_buff *skb,
>>> struct net_device *dev)
>>>         int err;
>>>         struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
>>>         bool kick = !skb->xmit_more;
>>> +       bool use_napi = sq->napi.weight;
>>>         /* Free up any pending old buffers before queueing new ones. */
>>> -       free_old_xmit_skbs(sq);
>>> +       if (!use_napi)
>>> +               free_old_xmit_skbs(sq);
>>
>>
>> I'm not sure this is best or even correct. Consider we clean xmit packets
>> speculatively in virtnet_poll_tx(), we need call free_old_xmit_skbs()
>> unconditionally. This can also help to reduce the possible of napi
>> rescheduling in virtnet_poll_tx().
>
> Because of the use of trylock there. Absolutely, thanks! Perhaps I should
> only use trylock in the opportunistic clean path from the rx softirq and
> full locking in the tx softirq.
>
> I previously observed that cleaning here would, counterintuitively,
> reduce efficiency. It reverted the improvements of cleaning transmit
> completions from the receive softirq. Going through my data, I did
> not observe this regression anymore on the latest patchset.
>
> Let me test again, with and without the new
> virtqueue_enable_cb_delayed patch. Perhaps that made a
> difference.

Neither cleaning in start_xmit nor converting the napi tx trylock to lock
shows a significant impact on loadtests, whether cpu affine or not.

I'll make both changes, as the first reduces patch size and code complexity
and the second is a more obviously correct codepath than than with trylock.

To be clear, the variant called from the rx napi handler will still
opportunistically use trylock.