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

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Date:   Mon, 10 Sep 2018 21:14:08 -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

> >> 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.
>
> Yes, maybe we can enable tx queue when napi weight is zero in
> virtnet_poll_tx().

Yes, that precaution should resolve that edge case.

> Let me do more stress test on this.
>
> >
> >>> 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.
>
> I get this.
>
> >
> > 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.
>
> I wonder maybe we can use ethtool_coalesce definition. E.g usecs = 0 &&
> max_frame = 0 means no interrupt? Just a thought, I'm ok with either.

Come to think of it, max_frames 0 is no worse than max_frames
UINT_MAX. Sure, let's do that.