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().
Cheers,
Paolo
> -----Original Message-----
> From: Paolo Abeni [mailto:[email protected]]
> Sent: Thursday, February 29, 2024 7:13 PM
> To: wangyunjian <[email protected]>; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]
> Cc: [email protected]; [email protected];
> [email protected]; [email protected];
> [email protected]; xudingke <[email protected]>; liwei (DT)
> <[email protected]>
> 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.
OK, I will update it, thanks.
>
> > + 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???
The vhost_net_rx_peek_head_len function obtains the packet length
but does not consume it. The packet is still in the ring. The later peek
will reuse it.
>
> I think the 'ring produce' part should be moved into tun_do_read().
Thank you for your suggestion. I will consider that.
>
> Cheers,
>
> Paolo
> -----Original Message-----
> From: Paolo Abeni [mailto:[email protected]]
> Sent: Thursday, February 29, 2024 7:13 PM
> To: wangyunjian <[email protected]>; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]
> Cc: [email protected]; [email protected];
> [email protected]; [email protected];
> [email protected]; xudingke <[email protected]>; liwei (DT)
> <[email protected]>
> 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