netdev - Re: [PATCH net-next] virtio_net: force_napi

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Message-ID: <CAF=yD-JmhKHhXcUdfm39t=OV+5VMT90XdGWaaKSWK+kG5UBh8Q@mail.gmail.com>
Date:   Tue, 24 Jul 2018 10:01:39 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     caleb.raitto@...il.com, Jason Wang <jasowang@...hat.com>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Caleb Raitto <caraitto@...gle.com>
Subject: Re: [PATCH net-next] virtio_net: force_napi_tx module param.

On Tue, Jul 24, 2018 at 6:44 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Jul 23, 2018 at 04:11:19PM -0700, Caleb Raitto wrote:
> > From: Caleb Raitto <caraitto@...gle.com>
> >
> > The driver disables tx napi if it's not certain that completions will
> > be processed affine with tx service.
> >
> > Its heuristic doesn't account for some scenarios where it is, such as
> > when the queue pair count matches the core but not hyperthread count.
> >
> > Allow userspace to override the heuristic. This is an alternative
> > solution to that in the linked patch. That added more logic in the
> > kernel for these cases, but the agreement was that this was better left
> > to user control.
> >
> > Do not expand the existing napi_tx variable to a ternary value,
> > because doing so can break user applications that expect
> > boolean ('Y'/'N') instead of integer output. Add a new param instead.
> >
> > Link: https://patchwork.ozlabs.org/patch/725249/
> > Acked-by: Willem de Bruijn <willemb@...gle.com>
> > Acked-by: Jon Olson <jonolson@...gle.com>
> > Signed-off-by: Caleb Raitto <caraitto@...gle.com>
>
> Is there a reason the same rule should apply to all devices?
> If not shouldn't this be an ethtool option?

It not very likely that a guest will have multiple virtio_net devices,
some of which have affinity_hint_set and some do not?

I'd really rather not add the extra option at all, but remove
the affinity_hint_set requirement for now. Without more data,
I understand the concern about cacheline bouncing if napi-tx
would becomes the default at some point and we don't have
data on this by then. But while it isn't default and a user has to
opt in to napi-tx to try it that seems enough guardrail to me.

The original reason was lack of data on whether napi-tx may suffer
from cache invalidations when tx and rx softirq are on different cpus
and we enable tx descriptor cleaning from the rx handler (i.e., on ACK).
>From those initial numbers it seemed to be a win even with those
invalidations.

  https://patchwork.ozlabs.org/patch/746232/

In lieu of removing the affinity_hint_set, this boolean is the least amount
of both new interface and implementation to allow experimentation. We
can easily leave it as a noop eventually when we are confident that
napi-tx can be enabled even without affinity. By comparison, an ethtool
param would be quite a bit of new logic.