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

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Date:   Thu, 13 Sep 2018 23:40:51 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        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 Thu, Sep 13, 2018 at 11:27 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
>
> On 2018年09月13日 22:58, Willem de Bruijn wrote:
> > On Thu, Sep 13, 2018 at 5:02 AM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >>
> >> On 2018年09月13日 07:27, Willem de Bruijn wrote:
> >>> On Wed, Sep 12, 2018 at 3:11 PM Willem de Bruijn
> >>> <willemdebruijn.kernel@...il.com> wrote:
> >>>> On Wed, Sep 12, 2018 at 2:16 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> >>>>>
> >>>>> On 9/12/2018 11:07 AM, Willem de Bruijn wrote:
> >>>>>> On Wed, Sep 12, 2018 at 1:42 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> >>>>>>>
> >>>>>>> On 9/9/2018 3:44 PM, 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.
> >>>>>>> Humm, would not a private ethtool flag to switch TX NAPI on/off be more
> >>>>>>> appropriate rather than use the coalescing configuration API here?
> >>>>>> What do you mean by private ethtool flag? A new field in ethtool
> >>>>>> --features (-k)?
> >>>>> I meant using ethtool_drvinfo::n_priv_flags, ETH_SS_PRIV_FLAGS and then
> >>>>> ETHTOOL_GFPFLAGS and ETHTOOL_SPFLAGS to control the toggling of that
> >>>>> private flag. mlx5 has a number of privates flags for instance.
> >>>> Interesting, thanks! I was not at all aware of those ethtool flags.
> >>>> Am having a look. It definitely looks promising.
> >>> Okay, I made that change. That is indeed much cleaner, thanks.
> >>> Let me send the patch, initially as RFC.
> >>>
> >>> I've observed one issue where if we toggle the flag before bringing
> >>> up the device, it hits a kernel BUG at include/linux/netdevice.h:515
> >>>
> >>>           BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
> >> This reminds me that we need to check netif_running() before trying to
> >> enable and disable tx napi in ethtool_set_coalesce().
> > The first iteration of my patch checked IFF_UP and effectively
> > only allowed the change when not running. What do you mean
> > by need to check?
>
> I mean if device is not up, there's no need to toggle napi state and tx
> lock.
>
> >
> > And to respond to the other follow-up notes at once:
> >
> >> Consider we may have interrupt moderation in the future, I tend to use
> >> set_coalesce. Otherwise we may need two steps to enable moderation:
> >>
> >> - tx-napi on
> >> - set_coalesce
> > FWIW, I don't care strongly whether we do this through coalesce or priv_flags.
>
> Ok.

Since you prefer coalesce, let's go with that (and a revision of your
latest patch).

>
> >>> +                     if (!napi_weight)
> >>> +                             virtqueue_enable_cb(vi->sq[i].vq);
> >> I don't get why we need to disable enable cb here.
> > To avoid entering no-napi mode with too few descriptors to
> > make progress and no way to get out of that state. This is a
> > pretty crude attempt at handling that, admittedly.
>
> But in this case, we will call enable_cb_delayed() and we will finally
> get a interrupt?

Right. It's a bit of a roundabout way to ensure that
netif_tx_wake_queue and thus eventually free_old_xmit_skbs are called.
It might make more sense to just wake the device without going through
an interrupt.