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

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Message-ID: <CAF=yD-+Aqe=9GbKGo_n748D97W2rJHdsYL+cay1gyR4eA2Hc=w@mail.gmail.com>
Date:   Wed, 12 Sep 2018 15:11:25 -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 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.

> > Configurable napi-tx is not a common feature across devices. We really
> > want virtio-net to also just convert to napi-tx as default, but need a
> > way to gradually convert with application opt-out if some workloads
> > see regressions.
>
> The rationale makes sense, no questions about it.
>
> > There is prior art in interpreting coalesce values as
> > more than a direct mapping to usec. The e1000 is one example.
> >
>
> Looked at both e1000 and e1000e and they both have a similar programming
> of the HW's interrupt target rate register, which is relevant to
> interrupt coalescing, what part of these drivers do you see as doing
> something not quite coalescing related?

It's all coalescing related, for sure. e1000_set_coalesce just does not
translate the tx-usecs values into microsecond latency directly.

It modifies both the interrupt throttle rate adapter->itr and interrupt mode
adapter->itr_setting, which are initially set in e1000_check_options from
module param InterruptThrottleRate.

Value 0 disables interrupt moderation. 1 and 3 program a dynamic mode.
2 is an illegal value as is 5..9. 10..10000 converts from usec to interrupt
rate/sec.

I took tx-napi to be a similar interrupt related option as, say, dynamic
conservative mode interrupt moderation.