Re: [PATCH bpf-next v2 06/10] xsk: propagate napi_id to XDP socket Rx path

From: "Michael S. Tsirkin" <mst@redhat.com>
To: "Björn Töpel" <bjorn.topel@intel.com>
Cc: "Björn Töpel" <bjorn.topel@gmail.com>,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	magnus.karlsson@intel.com, ast@kernel.org, daniel@iogearbox.net,
	maciej.fijalkowski@intel.com, sridhar.samudrala@intel.com,
	jesse.brandeburg@intel.com, qi.z.zhang@intel.com,
	kuba@kernel.org, edumazet@google.com, jonathan.lemon@gmail.com,
	maximmi@nvidia.com, intel-wired-lan@lists.osuosl.org,
	netanel@amazon.com, akiyano@amazon.com,
	michael.chan@broadcom.com, sgoutham@marvell.com,
	ioana.ciornei@nxp.com, ruxandra.radulescu@nxp.com,
	thomas.petazzoni@bootlin.com, mcroce@microsoft.com,
	saeedm@nvidia.com, tariqt@nvidia.com, aelior@marvell.com,
	ecree@solarflare.com, ilias.apalodimas@linaro.org,
	grygorii.strashko@ti.com, sthemmin@microsoft.com,
	kda@linux-powerpc.org
Subject: Re: [PATCH bpf-next v2 06/10] xsk: propagate napi_id to XDP socket Rx path
Date: Mon, 16 Nov 2020 08:55:56 -0500	[thread overview]
Message-ID: <20201116085548-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <585b011f-0817-a684-d1db-125bb55741fe@intel.com>

On Mon, Nov 16, 2020 at 02:24:56PM +0100, Björn Töpel wrote:
> On 2020-11-16 13:42, Michael S. Tsirkin wrote:
> > On Mon, Nov 16, 2020 at 01:01:40PM +0100, BjÃ¶rn TÃ¶pel wrote:
> > > 
> > > On 2020-11-16 12:55, Michael S. Tsirkin wrote:
> > > > On Mon, Nov 16, 2020 at 12:04:12PM +0100, BjÃfÂ¶rn TÃfÂ¶pel wrote:
> > > > > From: BjÃfÂ¶rn TÃfÂ¶pel <bjorn.topel@intel.com>
> > > > > 
> > > > > Add napi_id to the xdp_rxq_info structure, and make sure the XDP
> > > > > socket pick up the napi_id in the Rx path. The napi_id is used to find
> > > > > the corresponding NAPI structure for socket busy polling.
> > > > > 
> > > > > Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> > > > > Signed-off-by: BjÃfÂ¶rn TÃfÂ¶pel <bjorn.topel@intel.com>
> > > > 
> > > > A bunch of drivers just pass in 0. could you explain when
> > > > is that ok? how bad is it if the wrong id is used?
> > > > 
> > > 
> > > If zero is passed, which is a non-valid NAPI_ID, busy-polling will never
> > > be performed.
> > > 
> > > Depending on the structure of the driver, napi might or might not be
> > > initialized (napi_id != 0) or even available. When it wasn't obvious, I
> > > simply set it to zero.
> > > 
> > > So, short; No harm if zero, but busy-polling cannot be used in an XDP
> > > context.
> > > 
> > > 
> > > [...]
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index 21b71148c532..d71fe41595b7 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -1485,7 +1485,7 @@ static int virtnet_open(struct net_device *dev)
> > > > >    			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> > > > >    				schedule_delayed_work(&vi->refill, 0);
> > > > > -		err = xdp_rxq_info_reg(&vi->rq[i].xdp_rxq, dev, i);
> > > > > +		err = xdp_rxq_info_reg(&vi->rq[i].xdp_rxq, dev, i, 0);
> > > > >    		if (err < 0)
> > > > >    			return err;
> > > > 
> > > > Should this be rq.napi.napi_id ?
> > > > 
> > > 
> > > Yes, if rq.napi.napi_id is valid here! Is it?
> > 
> > What initializes it? netif_napi_add? Then I think yes, it's
> > initialized for all queues ...
> > Needs to be tested naturally.
> > 
> 
> Yeah, netid_napi_add does the id generation.
> 
> My idea was that driver would gradually move to a correct NAPI id (given
> that it's hard to test w/o HW. Virtio however is simpler to test. :-)
> 
> 
> Björn

tun too ;)