On Mon, Nov 9, 2020 at 10:06 PM John Fastabend <[email protected]> wrote:
>
> Magnus Karlsson wrote:
> > From: Magnus Karlsson <[email protected]>
> >
> > Introduce batched descriptor interfaces in the xsk core code for the
> > Tx path to be used in the driver to write a code path with higher
> > performance. This interface will be used by the i40e driver in the
> > next patch. Though other drivers would likely benefit from this new
> > interface too.
> >
> > Note that batching is only implemented for the common case when
> > there is only one socket bound to the same device and queue id. When
> > this is not the case, we fall back to the old non-batched version of
> > the function.
> >
> > Signed-off-by: Magnus Karlsson <[email protected]>
> > ---
> > include/net/xdp_sock_drv.h | 7 ++++
> > net/xdp/xsk.c | 43 ++++++++++++++++++++++
> > net/xdp/xsk_queue.h | 89
> > +++++++++++++++++++++++++++++++++++++++-------
> > 3 files changed, 126 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
> > index 5b1ee8a..4e295541 100644
> > --- a/include/net/xdp_sock_drv.h
> > +++ b/include/net/xdp_sock_drv.h
> > @@ -13,6 +13,7 @@
> >
> > void xsk_tx_completed(struct xsk_buff_pool *pool, u32 nb_entries);
> > bool xsk_tx_peek_desc(struct xsk_buff_pool *pool, struct xdp_desc *desc);
> > +u32 xsk_tx_peek_release_desc_batch(struct xsk_buff_pool *pool, struct
> > xdp_desc *desc, u32 max);
> > void xsk_tx_release(struct xsk_buff_pool *pool);
> > struct xsk_buff_pool *xsk_get_pool_from_qid(struct net_device *dev,
> > u16 queue_id);
> > @@ -128,6 +129,12 @@ static inline bool xsk_tx_peek_desc(struct
> > xsk_buff_pool *pool,
> > return false;
> > }
> >
> > +static inline u32 xsk_tx_peek_release_desc_batch(struct xsk_buff_pool
> > *pool, struct xdp_desc *desc,
> > + u32 max)
> > +{
> > + return 0;
> > +}
> > +
> > static inline void xsk_tx_release(struct xsk_buff_pool *pool)
> > {
> > }
> > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > index b71a32e..dd75b5f 100644
> > --- a/net/xdp/xsk.c
> > +++ b/net/xdp/xsk.c
> > @@ -332,6 +332,49 @@ bool xsk_tx_peek_desc(struct xsk_buff_pool *pool,
> > struct xdp_desc *desc)
> > }
> > EXPORT_SYMBOL(xsk_tx_peek_desc);
> >
> > +u32 xsk_tx_peek_release_desc_batch(struct xsk_buff_pool *pool, struct
> > xdp_desc *descs,
> > + u32 max_entries)
> > +{
> > + struct xdp_sock *xs;
> > + u32 nb_pkts;
> > +
> > + rcu_read_lock();
> > + if (!list_is_singular(&pool->xsk_tx_list)) {
> > + /* Fallback to the non-batched version */
> > + rcu_read_unlock();
> > + return xsk_tx_peek_desc(pool, &descs[0]) ? 1 : 0;
> > + }
> > +
> > + xs = list_first_or_null_rcu(&pool->xsk_tx_list, struct xdp_sock,
> > tx_list);
>
> I'm not seeing how we avoid the null check here? Can you add a comment on why
> this
> is safe? I see the bind/unbind routines is it possible to unbind while this is
> running or do we have some locking here.
You are correct. The entry can disappear between list_is_singluar and
list_first_or_null_rcu. There are 3 possibilities at this point:
0 entries: as you point out, we need to test for this and exit since
the socket does not exist anymore.
1 entry: everything is working as expected.
>1 entry: we only process the first socket in the list. This is fine since this
>can only happen when we add a second socket to the list and the next time we
>enter this function list_is_singular() will not be true anymore, so we will
>use the fallback version that will process packets from all sockets. So the
>only thing that will happen in this rare case is that the start of processing
>for the second socket is delayed ever so slightly.
In summary, I will add a test for !xs and exit in that case.
> > +
> > + nb_pkts = xskq_cons_peek_desc_batch(xs->tx, descs, pool, max_entries);
> > + if (!nb_pkts) {
> > + xs->tx->queue_empty_descs++;
> > + goto out;
> > + }
> > +
> > + /* This is the backpressure mechanism for the Tx path. Try to
> > + * reserve space in the completion queue for all packets, but
> > + * if there are fewer slots available, just process that many
> > + * packets. This avoids having to implement any buffering in
> > + * the Tx path.
> > + */
> > + nb_pkts = xskq_prod_reserve_addr_batch(pool->cq, descs, nb_pkts);
> > + if (!nb_pkts)
> > + goto out;
> > +
> > + xskq_cons_release_n(xs->tx, nb_pkts);
> > + __xskq_cons_release(xs->tx);
> > + xs->sk.sk_write_space(&xs->sk);
>
> Can you move the out label here? Looks like nb_pkts = 0 in all cases
> where goto out is used.
Nice simplification. Will fix.
Thanks: Magnus
> > + rcu_read_unlock();
> > + return nb_pkts;
> > +
> > +out:
> > + rcu_read_unlock();
> > + return 0;
> > +}
> > +EXPORT_SYMBOL(xsk_tx_peek_release_desc_batch);
> > +
> > static int xsk_wakeup(struct xdp_sock *xs, u8 flags)
> > {
> > struct net_device *dev = xs->dev;
>
> [...]
>
> Other than above question LGTM.
>
> Thanks,
> John
