Re: [PATCH net] xsk: correct tx_ring_empty_descs count statistics

[Wang Liang]

在 2025/4/1 6:03, Stanislav Fomichev 写道:
> On 03/31, Stanislav Fomichev wrote:
> > On 03/29, Wang Liang wrote:
> > > The tx_ring_empty_descs count may be incorrect, when set the XDP_TX_RING
> > > option but do not reserve tx ring. Because xsk_poll() try to wakeup the
> > > driver by calling xsk_generic_xmit() for non-zero-copy mode. So the
> > > tx_ring_empty_descs count increases once the xsk_poll()is called:
> > >
> > >    xsk_poll
> > >      xsk_generic_xmit
> > >        __xsk_generic_xmit
> > >          xskq_cons_peek_desc
> > >            xskq_cons_read_desc
> > >              q->queue_empty_descs++;
> > >
> > > To avoid this count error, add check for tx descs before send msg in poll.
> > >
> > > Fixes: df551058f7a3 ("xsk: Fix crash in poll when device does not support ndo_xsk_wakeup")
> > > Signed-off-by: Wang Liang <wangliang74@xxxxxxxxxx>
> > Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>
> Hmm, wait, I stumbled upon xskq_has_descs again and it looks only at
> cached prod/cons. How is it supposed to work when the actual tx
> descriptor is posted? Is there anything besides xskq_cons_peek_desc from
> __xsk_generic_xmit that refreshes cached_prod?


Yes, you are right!

How about using xskq_cons_nb_entries() to check free descriptors?

Like this:


diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index e5d104ce7b82..babb7928d335 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -993,7 +993,7 @@ static __poll_t xsk_poll(struct file *file, struct 
socket *sock,
        if (pool->cached_need_wakeup) {
                if (xs->zc)
                        xsk_wakeup(xs, pool->cached_need_wakeup);
-               else if (xs->tx)
+               else if (xs->tx && xskq_cons_nb_entries(xs->tx, 1))
                        /* Poll needs to drive Tx also in copy mode */
                        xsk_generic_xmit(sk);
        }