RE: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support

[wangyunjian]

> -----Original Message-----
> From: Paolo Abeni [mailto:pabeni@xxxxxxxxxx]
> Sent: Thursday, February 29, 2024 7:13 PM
> To: wangyunjian <wangyunjian@xxxxxxxxxx>; mst@xxxxxxxxxx;
> willemdebruijn.kernel@xxxxxxxxx; jasowang@xxxxxxxxxx; kuba@xxxxxxxxxx;
> bjorn@xxxxxxxxxx; magnus.karlsson@xxxxxxxxx; maciej.fijalkowski@xxxxxxxxx;
> jonathan.lemon@xxxxxxxxx; davem@xxxxxxxxxxxxx
> Cc: bpf@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> virtualization@xxxxxxxxxxxxxxx; xudingke <xudingke@xxxxxxxxxx>; liwei (DT)
> <liwei395@xxxxxxxxxx>
> Subject: Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support
> 
> On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> > @@ -2661,6 +2776,54 @@ static int tun_ptr_peek_len(void *ptr)
> >  	}
> >  }
> >
> > +static void tun_peek_xsk(struct tun_file *tfile) {
> > +	struct xsk_buff_pool *pool;
> > +	u32 i, batch, budget;
> > +	void *frame;
> > +
> > +	if (!ptr_ring_empty(&tfile->tx_ring))
> > +		return;
> > +
> > +	spin_lock(&tfile->pool_lock);
> > +	pool = tfile->xsk_pool;
> > +	if (!pool) {
> > +		spin_unlock(&tfile->pool_lock);
> > +		return;
> > +	}
> > +
> > +	if (tfile->nb_descs) {
> > +		xsk_tx_completed(pool, tfile->nb_descs);
> > +		if (xsk_uses_need_wakeup(pool))
> > +			xsk_set_tx_need_wakeup(pool);
> > +	}
> > +
> > +	spin_lock(&tfile->tx_ring.producer_lock);
> > +	budget = min_t(u32, tfile->tx_ring.size, TUN_XDP_BATCH);
> > +
> > +	batch = xsk_tx_peek_release_desc_batch(pool, budget);
> > +	if (!batch) {
> 
> This branch looks like an unneeded "optimization". The generic loop below
> should have the same effect with no measurable perf delta - and smaller code.
> Just remove this.
> 
> > +		tfile->nb_descs = 0;
> > +		spin_unlock(&tfile->tx_ring.producer_lock);
> > +		spin_unlock(&tfile->pool_lock);
> > +		return;
> > +	}
> > +
> > +	tfile->nb_descs = batch;
> > +	for (i = 0; i < batch; i++) {
> > +		/* Encode the XDP DESC flag into lowest bit for consumer to differ
> > +		 * XDP desc from XDP buffer and sk_buff.
> > +		 */
> > +		frame = tun_xdp_desc_to_ptr(&pool->tx_descs[i]);
> > +		/* The budget must be less than or equal to tx_ring.size,
> > +		 * so enqueuing will not fail.
> > +		 */
> > +		__ptr_ring_produce(&tfile->tx_ring, frame);
> > +	}
> > +	spin_unlock(&tfile->tx_ring.producer_lock);
> > +	spin_unlock(&tfile->pool_lock);
> 
> More related to the general design: it looks wrong. What if
> get_rx_bufs() will fail (ENOBUF) after successful peeking? With no more
> incoming packets, later peek will return 0 and it looks like that the
> half-processed packets will stay in the ring forever???
> 
> I think the 'ring produce' part should be moved into tun_do_read().

Currently, the vhost-net obtains a batch descriptors/sk_buffs from the
ptr_ring and enqueue the batch descriptors/sk_buffs to the virtqueue'queue,
and then consumes the descriptors/sk_buffs from the virtqueue'queue in
sequence. As a result, TUN does not know whether the batch descriptors have
been used up, and thus does not know when to return the batch descriptors.

So, I think it's reasonable that when vhost-net checks ptr_ring is empty,
it calls peek_len to get new xsk's descs and return the descriptors.

Thanks
> 
> Cheers,
> 
> Paolo