On Wed, 30 Nov 2016 07:56:26 -0800
Eric Dumazet <eric.duma...@gmail.com> wrote:
> On Wed, 2016-11-30 at 12:38 +0100, Jesper Dangaard Brouer wrote:
> > I've played with a somewhat similar patch (from Achiad Shochat) for
> > mlx5 (attached). While it gives huge improvements, the problem I ran
> > into was that; TX performance became a function of the TX completion
> > time/interrupt and could easily be throttled if configured too
> > high/slow.
> >
> > Can your patch be affected by this too?
>
> Like all TX business, you should know this Jesper.
> No need to constantly remind us something very well known.
Don't take is as critique Eric. I was hoping your patch would have
solved this issue of being sensitive to TX completion adjustments. You
usually have good solutions for difficult issues. I basically rejected
Achiad's approach/patch because it was too sensitive to these kind of
adjustments.
> > On Mon, 28 Nov 2016 22:58:36 -0800 Eric Dumazet <eric.duma...@gmail.com>
> > wrote:
[...]
> >
> > These +75% number is pktgen without "burst", and definitely show that
> > your patch activate xmit_more.
> > What is the pps performance number when using pktgen "burst" option?
>
> About the same really. About all packets now get the xmit_more effect.
Perfect!
> > [...]
> > > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> > > b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> > > index 4b597dca5c52..affebb435679 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> > > +++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> > [...]
> > > -static inline bool mlx4_en_is_tx_ring_full(struct mlx4_en_tx_ring *ring)
> > > +static inline bool mlx4_en_is_tx_ring_full(const struct mlx4_en_tx_ring
> > > *ring)
> > > {
> > > - return ring->prod - ring->cons > ring->full_size;
> > > + return READ_ONCE(ring->prod) - READ_ONCE(ring->cons) > ring->full_size;
> > > }
> > [...]
> >
> > > @@ -1033,6 +1058,14 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb,
> > > struct net_device *dev)
> > > }
> > > send_doorbell = !skb->xmit_more || netif_xmit_stopped(ring->tx_queue);
> > >
> > > + /* Doorbell avoidance : We can omit doorbell if we know a TX completion
> > > + * will happen shortly.
> > > + */
> > > + if (send_doorbell &&
> > > + dev->doorbell_opt &&
> > > + (s32)(READ_ONCE(ring->prod_bell) - READ_ONCE(ring->ncons)) > 0)
> >
> > It would be nice with a function call with an appropriate name, instead
> > of an open-coded queue size check. I'm also confused by the "ncons" name.
> >
> > > + send_doorbell = false;
> > > +
> > [...]
> >
> > > diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> > > b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> > > index 574bcbb1b38f..c3fd0deda198 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> > > +++ b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> > > @@ -280,6 +280,7 @@ struct mlx4_en_tx_ring {
> > > */
> > > u32 last_nr_txbb;
> > > u32 cons;
> > > + u32 ncons;
> >
> > Maybe we can find a better name than "ncons" ?
>
> Thats because 'cons' in this driver really means 'old cons'
>
> and new cons = old cons + last_nr_txbb;
It was not clear to me that "n" meant "new". And also not clear that
this drive have an issue of "cons" (consumer) is tracking "old" cons.
> > > unsigned long wake_queue;
> > > struct netdev_queue *tx_queue;
> > > u32 (*free_tx_desc)(struct mlx4_en_priv *priv,
> > > @@ -290,6 +291,7 @@ struct mlx4_en_tx_ring {
> > >
> > > /* cache line used and dirtied in mlx4_en_xmit() */
> > > u32 prod ____cacheline_aligned_in_smp;
> > > + u32 prod_bell;
> >
> > Good descriptive variable name.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
