[Intel-wired-lan] [PATCH bpf-next v3 4/5] xsk: introduce batched Tx descriptor interfaces

[Magnus Karlsson]

On Tue, Nov 17, 2020 at 8:07 PM John Fastabend <john.fastabend at gmail.com> wrote:
>
> Magnus Karlsson wrote:
> > From: Magnus Karlsson <magnus.karlsson at intel.com>
> >
> > 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 <magnus.karlsson at intel.com>
> > ---
> >  include/net/xdp_sock_drv.h |  7 ++++
> >  net/xdp/xsk.c              | 57 +++++++++++++++++++++++++++++
> >  net/xdp/xsk_queue.h        | 89 +++++++++++++++++++++++++++++++++++++++-------
> >  3 files changed, 140 insertions(+), 13 deletions(-)
> >
>
> Acked-by: John Fastabend <john.fastabend at gmail.com>
>
> > +
> > +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 */
>
> I'm going to ask even though I believe its correct.
>
> If we fallback here and then an entry is added to the list while we are
> in the fallback logic everything should still be OK, correct?

Yes, the fallback function can handle all cases.

> > +             rcu_read_unlock();
> > +             return xsk_tx_peek_release_fallback(pool, descs, max_entries);
> > +     }
> > +
> > +     xs = list_first_or_null_rcu(&pool->xsk_tx_list, struct xdp_sock, tx_list);
> > +     if (!xs) {
> > +             nb_pkts = 0;
> > +             goto out;
> > +     }
> > +
> > +     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);
> > +
> > +out:
> > +     rcu_read_unlock();
> > +     return nb_pkts;
> > +}
> > +EXPORT_SYMBOL(xsk_tx_peek_release_desc_batch);
> > +