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Message-ID: <CACGkMEvHVxZcp2efz5EEW96szHBeU0yAfkLy7qSQnVZmxm4GLQ@mail.gmail.com>
Date: Fri, 24 Jan 2025 09:14:54 +0800
From: Jason Wang <jasowang@...hat.com>
To: Joe Damato <jdamato@...tly.com>, Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org, 
	gerhard@...leder-embedded.com, leiyang@...hat.com, xuanzhuo@...ux.alibaba.com, 
	mkarsten@...terloo.ca, "Michael S. Tsirkin" <mst@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"open list:VIRTIO CORE AND NET DRIVERS" <virtualization@...ts.linux.dev>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC net-next v3 2/4] virtio_net: Prepare for NAPI to queue mapping

On Thu, Jan 23, 2025 at 10:47 AM Joe Damato <jdamato@...tly.com> wrote:
>
> On Thu, Jan 23, 2025 at 10:40:43AM +0800, Jason Wang wrote:
> > On Thu, Jan 23, 2025 at 1:41 AM Joe Damato <jdamato@...tly.com> wrote:
> > >
> > > On Wed, Jan 22, 2025 at 02:12:46PM +0800, Jason Wang wrote:
> > > > On Wed, Jan 22, 2025 at 3:11 AM Joe Damato <jdamato@...tly.com> wrote:
> > > > >
> > > > > Slight refactor to prepare the code for NAPI to queue mapping. No
> > > > > functional changes.
> > > > >
> > > > > Signed-off-by: Joe Damato <jdamato@...tly.com>
> > > > > Reviewed-by: Gerhard Engleder <gerhard@...leder-embedded.com>
> > > > > Tested-by: Lei Yang <leiyang@...hat.com>
> > > > > ---
> > > > >  v2:
> > > > >    - Previously patch 1 in the v1.
> > > > >    - Added Reviewed-by and Tested-by tags to commit message. No
> > > > >      functional changes.
> > > > >
> > > > >  drivers/net/virtio_net.c | 10 ++++++++--
> > > > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index 7646ddd9bef7..cff18c66b54a 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -2789,7 +2789,8 @@ static void skb_recv_done(struct virtqueue *rvq)
> > > > >         virtqueue_napi_schedule(&rq->napi, rvq);
> > > > >  }
> > > > >
> > > > > -static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
> > > > > +static void virtnet_napi_do_enable(struct virtqueue *vq,
> > > > > +                                  struct napi_struct *napi)
> > > > >  {
> > > > >         napi_enable(napi);
> > > >
> > > > Nit: it might be better to not have this helper to avoid a misuse of
> > > > this function directly.
> > >
> > > Sorry, I'm probably missing something here.
> > >
> > > Both virtnet_napi_enable and virtnet_napi_tx_enable need the logic
> > > in virtnet_napi_do_enable.
> > >
> > > Are you suggesting that I remove virtnet_napi_do_enable and repeat
> > > the block of code in there twice (in virtnet_napi_enable and
> > > virtnet_napi_tx_enable)?
> >
> > I think I miss something here, it looks like virtnet_napi_tx_enable()
> > calls virtnet_napi_do_enable() directly.
> >
> > I would like to know why we don't call netif_queue_set_napi() for TX NAPI here?
>
> Please see both the cover letter and the commit message of the next
> commit which addresses this question.
>
> TX-only NAPIs do not have NAPI IDs so there is nothing to map.

Interesting, but I have more questions

1) why need a driver to know the NAPI implementation like this?

2) does NAPI know (or why it needs to know) whether or not it's a TX
or not? I only see the following code in napi_hash_add():

static void napi_hash_add(struct napi_struct *napi)
{
        unsigned long flags;

        if (test_bit(NAPI_STATE_NO_BUSY_POLL, &napi->state))
                return;

...

        __napi_hash_add_with_id(napi, napi_gen_id);

        spin_unlock_irqrestore(&napi_hash_lock, flags);
}

It seems it only matters with NAPI_STATE_NO_BUSY_POLL.

And if NAPI knows everything, should it be better to just do the
linking in napi_enable/disable() instead of letting each driver do it
by itself?

Thanks

>


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