Re: [PATCH bpf-next] xsk: add test for tx_writeable to batched path

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Tue, Dec 14, 2021 at 3:28 PM Maciej Fijalkowski
<maciej.fijalkowski@intel.com> wrote:
>
> On Tue, Dec 14, 2021 at 11:26:47AM +0100, Magnus Karlsson wrote:
> > From: Magnus Karlsson <magnus.karlsson@intel.com>
> >
> > Add a test for the tx_writeable condition to the batched Tx processing
> > path. This test is in the skb and non-batched code paths but not in the
> > batched code path. So add it there. This test makes sure that a
> > process is not woken up until there are a sufficiently large number of
> > free entries in the Tx ring. Currently, any driver using the batched
> > interface will be woken up even if there is only one free entry,
> > impacting performance negatively.
>
> I gave this patch a shot on ice driver with the Tx batching patch that i'm
> about to send which is using the xsk_tx_peek_release_desc_batch(). I ran
> the 2 core setup with no busy poll and it turned out that this change has
> a negative impact on performance - it degrades by 5%.
>
> After a short chat with Magnus he said it's due to the touch to the global
> state of a ring that xsk_tx_writeable() is doing.
>
> So maintainers, please do not apply this yet, we'll come up with a
> solution.
>
> Also, should this be sent to bpf tree (not bpf-next) ?

It is just a performance fix, so I would say bpf-next.

> Thanks!
>
> >
> > Fixes: 3413f04141aa ("xsk: Change the tx writeable condition")
> > Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
> > ---
> >  net/xdp/xsk.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > index 28ef3f4465ae..3772fcaa76ed 100644
> > --- a/net/xdp/xsk.c
> > +++ b/net/xdp/xsk.c
> > @@ -392,7 +392,8 @@ u32 xsk_tx_peek_release_desc_batch(struct xsk_buff_pool *pool, struct xdp_desc *
> >
> >       xskq_cons_release_n(xs->tx, nb_pkts);
> >       __xskq_cons_release(xs->tx);
> > -     xs->sk.sk_write_space(&xs->sk);
> > +     if (xsk_tx_writeable(xs))
> > +             xs->sk.sk_write_space(&xs->sk);
> >
> >  out:
> >       rcu_read_unlock();
> >
> > base-commit: d27a662290963a1cde26cdfdbac71a546c06e94a
> > --
> > 2.29.0
> >