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Message-ID: <223aeca6435342ec8a4d57c959c23303@huawei.com>
Date: Fri, 1 Mar 2024 11:45:52 +0000
From: wangyunjian <wangyunjian@...wei.com>
To: Paolo Abeni <pabeni@...hat.com>, "mst@...hat.com" <mst@...hat.com>,
"willemdebruijn.kernel@...il.com" <willemdebruijn.kernel@...il.com>,
"jasowang@...hat.com" <jasowang@...hat.com>, "kuba@...nel.org"
<kuba@...nel.org>, "bjorn@...nel.org" <bjorn@...nel.org>,
"magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
"maciej.fijalkowski@...el.com" <maciej.fijalkowski@...el.com>,
"jonathan.lemon@...il.com" <jonathan.lemon@...il.com>, "davem@...emloft.net"
<davem@...emloft.net>
CC: "bpf@...r.kernel.org" <bpf@...r.kernel.org>, "netdev@...r.kernel.org"
<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>, xudingke
<xudingke@...wei.com>, "liwei (DT)" <liwei395@...wei.com>
Subject: RE: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support
> -----Original Message-----
> From: Paolo Abeni [mailto:pabeni@...hat.com]
> Sent: Thursday, February 29, 2024 7:13 PM
> To: wangyunjian <wangyunjian@...wei.com>; mst@...hat.com;
> willemdebruijn.kernel@...il.com; jasowang@...hat.com; kuba@...nel.org;
> bjorn@...nel.org; magnus.karlsson@...el.com; maciej.fijalkowski@...el.com;
> jonathan.lemon@...il.com; davem@...emloft.net
> Cc: bpf@...r.kernel.org; netdev@...r.kernel.org;
> linux-kernel@...r.kernel.org; kvm@...r.kernel.org;
> virtualization@...ts.linux.dev; xudingke <xudingke@...wei.com>; liwei (DT)
> <liwei395@...wei.com>
> 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
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