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

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Message-ID: <CAF=yD-KhFpzb8APxZypu-C8rKVkUUKarA1fhB=9JFT7SXCpuXA@mail.gmail.com>
Date:   Wed, 12 Sep 2018 19:27:18 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     f.fainelli@...il.com
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, caleb.raitto@...il.com,
        Jason Wang <jasowang@...hat.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 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));