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

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Date:   Mon, 10 Sep 2018 09:35:27 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, caleb.raitto@...il.com,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Jon Olson (Google Drive)" <jonolson@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] virtio_net: ethtool tx napi configuration

On Mon, Sep 10, 2018 at 2:01 AM Jason Wang <jasowang@...hat.com> wrote:
>
>
>
> On 2018年09月10日 06:44, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > 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, use bit 10, well outside
> > the reasonable range of real interrupt moderation values.
> >
> > Changes are not atomic. The tx IRQ, napi BH and transmit path must
> > be quiesced when switching modes. Only allow changing this setting
> > when the device is down.
>
> I cook a fixup, and it looks works in my setup:
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index b320b6b14749..9181c3f2f832 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2204,10 +2204,17 @@ static int virtnet_set_coalesce(struct
> net_device *dev,
>                  return -EINVAL;
>
>          if (napi_weight ^ vi->sq[0].napi.weight) {
> -               if (dev->flags & IFF_UP)
> -                       return -EBUSY;
> -               for (i = 0; i < vi->max_queue_pairs; i++)
> +               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;

Thanks! It passes my simple stress test, too. Which consists of two
concurrent loops, one toggling the ethtool option, another running
TCP_RR.

> The only left case is the speculative tx polling in RX NAPI. I think we
> don't need to care in this case since it was not a must for correctness.

As long as the txq lock is held that will be a noop, anyway. The other
concurrent action is skb_xmit_done. It looks correct to me, but need
to think about it a bit. The tricky transition is coming out of napi without
having >= 2 + MAX_SKB_FRAGS clean descriptors. If the queue is
stopped it may deadlock transmission in no-napi mode.

> >
> > Link: https://patchwork.ozlabs.org/patch/948149/
> > Suggested-by: Jason Wang <jasowang@...hat.com>
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > ---
> >   drivers/net/virtio_net.c | 52 ++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 52 insertions(+)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 765920905226..b320b6b14749 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);
> > +
> >   static const unsigned long guest_offloads[] = {
> >       VIRTIO_NET_F_GUEST_TSO4,
> >       VIRTIO_NET_F_GUEST_TSO6,
> > @@ -2181,6 +2183,54 @@ 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)
> > +{
> > +     const struct ethtool_coalesce ec_default = {
> > +             .cmd = ETHTOOL_SCOALESCE,
> > +             .rx_max_coalesced_frames = 1,
>
> I think rx part is no necessary.

The definition of ethtool_coalesce has:

"* It is illegal to set both usecs and max_frames to zero as this
 * would cause interrupts to never be generated.  To disable
 * coalescing, set usecs = 0 and max_frames = 1."

I'd rather not diverge from this prescribed behavior unless there's
a strong reason.

On the related point in the other thread:

> Rethink about this, how about something like:
>
> - UINT_MAX: no tx interrupt
> - other value: tx interrupt with possible interrupt moderation

Okay, that will be simpler to configure.