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Message-ID: <7d478cb842e28094f4d6102e593e3de25ab27dfe.camel@redhat.com>
Subject: Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support
From: Paolo Abeni <pabeni@redhat.com>
To: Yunjian Wang <wangyunjian@huawei.com>, mst@redhat.com, 
	willemdebruijn.kernel@gmail.com, jasowang@redhat.com, kuba@kernel.org, 
	bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, 
	jonathan.lemon@gmail.com, davem@davemloft.net
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org,
 linux-kernel@vger.kernel.org,  kvm@vger.kernel.org,
 virtualization@lists.linux.dev, xudingke@huawei.com,  liwei395@huawei.com
Date: Thu, 29 Feb 2024 12:12:44 +0100
In-Reply-To: <1709118356-133960-1-git-send-email-wangyunjian@huawei.com>
References: <1709118356-133960-1-git-send-email-wangyunjian@huawei.com>
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On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> @@ -2661,6 +2776,54 @@ static int tun_ptr_peek_len(void *ptr)
>  	}
>  }
> =20
> +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 =3D 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 =3D min_t(u32, tfile->tx_ring.size, TUN_XDP_BATCH);
> +
> +	batch =3D 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 =3D 0;
> +		spin_unlock(&tfile->tx_ring.producer_lock);
> +		spin_unlock(&tfile->pool_lock);
> +		return;
> +	}
> +
> +	tfile->nb_descs =3D batch;
> +	for (i =3D 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 =3D 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().

Cheers,

Paolo