Re: [PATCH net-next V2] virtio_net: ethtool tx napi configuration

[Jason Wang <jasowang@redhat.com>]

On 2018年09月13日 23:20, Willem de Bruijn wrote:
> On Thu, Sep 13, 2018 at 1:40 AM Jason Wang <jasowang@redhat.com> wrote:
>> Implement ethtool .set_coalesce (-C) and .get_coalesce (-c) handlers.
>> Interrupt moderation is currently not supported, so these accept and
>> display the default settings of 0 usec and 1 frame.
>>
>> Toggle tx napi through a bit in tx-frames. So as to not interfere
>> with possible future interrupt moderation, value 1 means tx napi while
>> value 0 means not.
>>
>> To properly synchronize with the data path, tx napi is disabled and
>> tx lock is held when changing the value of napi weight. And two more
>> places that can access tx napi weight:
>>
>> - speculative tx polling in rx napi, we can leave it as is since it
>>    not a must for correctness.
>> - skb_xmit_done(), one more check of napi weight is added before
>>    trying to enable tx to avoid tx to be disabled forever if napi is
>>    disabled after skb_xmit_done() but before the napi
>>
>> Link: https://patchwork.ozlabs.org/patch/948149/
>> Suggested-by: Jason Wang <jasowang@redhat.com>
>> Signed-off-by: Willem de Bruijn <willemb@google.com>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>> Changes from V1:
>> - try to synchronize with datapath to allow changing mode when
>>    interface is up.
>> - use tx-frames 0 as to disable tx napi while tx-frames 1 to enable tx napi
>> ---
>>   drivers/net/virtio_net.c | 64 +++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 63 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 765920905226..6e70864f5899 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -66,6 +66,8 @@ DECLARE_EWMA(pkt_len, 0, 64)
>>
>>   #define VIRTNET_DRIVER_VERSION "1.0.0"
>>
>> +static const u32 ethtool_coalesce_napi_mask = (1UL << 10);
>> +
> This is no longer needed

Yes, will remove this.

>
>>   static const unsigned long guest_offloads[] = {
>>          VIRTIO_NET_F_GUEST_TSO4,
>>          VIRTIO_NET_F_GUEST_TSO6,
>> @@ -1444,7 +1446,10 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
>>
>>          virtqueue_napi_complete(napi, sq->vq, 0);
>>
>> -       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
>> +       /* Check napi.weight to avoid tx stall since it could be set
>> +        * to zero by ethtool after skb_xmit_done().
>> +        */
>> +       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS || !sq->napi.weight)
>>                  netif_tx_wake_queue(txq);
> I see. This assumes that the napi handler will always be called on
> conversion from napi to no-napi mode.
>
> That is safe to assume because if it isn't called (and will not call
> netif_tx_wake_queue) that implies that napi was not scheduled, and
> thus the tx interrupt was not suppressed and thus there was no tx
> completion work to be scheduled?

If it isn't called it means skb_xmit_done() wakeup tx directly instead 
of schedule tx. This could be a little bit early since there may be 
still lots of pending tx packets. But it doesn't harm, start_xmit() can 
handle this by re enable a delayed tx interrupt and disable TX.

But there's a bug, look like I need remove the check of 
(!sq->napi.weight) in the beginning of the function.

>
>>          return 0;
>> @@ -2181,6 +2186,61 @@ static int virtnet_get_link_ksettings(struct net_device *dev,
>>          return 0;
>>   }
>>
>> +static int virtnet_set_coalesce(struct net_device *dev,
>> +                               struct ethtool_coalesce *ec)
>> +{
>> +       struct ethtool_coalesce ec_default = {
>> +               .cmd = ETHTOOL_SCOALESCE,
>> +               .rx_max_coalesced_frames = 1,
>> +       };
>> +       struct virtnet_info *vi = netdev_priv(dev);
>> +       int i, napi_weight;
>> +
>> +       if (ec->tx_max_coalesced_frames > 1)
>> +               return -EINVAL;
>> +
>> +       ec_default.tx_max_coalesced_frames = ec->tx_max_coalesced_frames;
>> +       napi_weight = ec->tx_max_coalesced_frames ? NAPI_POLL_WEIGHT : 0;
>> +
>> +       /* disallow changes to fields not explicitly tested above */
>> +       if (memcmp(ec, &ec_default, sizeof(ec_default)))
>> +               return -EINVAL;
>> +
>> +       if (napi_weight ^ vi->sq[0].napi.weight) {
>> +               for (i = 0; i < vi->max_queue_pairs; i++) {
>> +                       struct netdev_queue *txq =
>> +                              netdev_get_tx_queue(vi->dev, i);
>> +
>> +                       virtnet_napi_tx_disable(&vi->sq[i].napi);
>> +                       __netif_tx_lock_bh(txq);
>> +                       vi->sq[i].napi.weight = napi_weight;
>> +                       __netif_tx_unlock_bh(txq);
>> +                       virtnet_napi_tx_enable(vi, vi->sq[i].vq,
>> +                                              &vi->sq[i].napi);
>> +               }
>> +       }
>> +
>> +       return 0;
>> +}
>> +
>> +static int virtnet_get_coalesce(struct net_device *dev,
>> +                               struct ethtool_coalesce *ec)
>> +{
>> +       struct ethtool_coalesce ec_default = {
>> +               .cmd = ETHTOOL_GCOALESCE,
>> +               .rx_max_coalesced_frames = 1,
>> +               .tx_max_coalesced_frames = 0,
> no need to explicitly initialize to 0 (unless you did this for
> documentation purposes, which is fine).

Yes.

Thanks

>> +       };
>> +       struct virtnet_info *vi = netdev_priv(dev);
>> +
>> +       memcpy(ec, &ec_default, sizeof(ec_default));
>> +
>> +       if (vi->sq[0].napi.weight)
>> +               ec->tx_max_coalesced_frames = 1;
>> +
>> +       return 0;
>> +}
>> +
>>   static void virtnet_init_settings(struct net_device *dev)
>>   {
>>          struct virtnet_info *vi = netdev_priv(dev);
>> @@ -2219,6 +2279,8 @@ static const struct ethtool_ops virtnet_ethtool_ops = {
>>          .get_ts_info = ethtool_op_get_ts_info,
>>          .get_link_ksettings = virtnet_get_link_ksettings,
>>          .set_link_ksettings = virtnet_set_link_ksettings,
>> +       .set_coalesce = virtnet_set_coalesce,
>> +       .get_coalesce = virtnet_get_coalesce,
>>   };
>>
>>   static void virtnet_freeze_down(struct virtio_device *vdev)
>> --
>> 2.17.1
>>