Re: [dpdk-dev] [PATCH v2 4/7] net/mlx4: merge Tx path functions

["Nélio Laranjeiro" <nelio.laranjeiro@6wind.com>]

On Thu, Oct 26, 2017 at 10:31:06AM +0000, Matan Azrad wrote:
> Hi Nelio
> 
> I think the memory barrier discussion is not relevant for this patch
> (if it will be relevant I will create new one).
> Please see my comments inline.

It was not my single comment.  There is also useless code like having
null segments in the packets which is not allowed on DPDK.

> Regarding this specific patch, I didn't see any comment from you, Are
> you agree with it? 
>  
> > -----Original Message-----
> > From: Nélio Laranjeiro [mailto:nelio.laranjeiro@6wind.com]
> > Sent: Wednesday, October 25, 2017 10:50 AM
> > To: Ophir Munk <ophirmu@mellanox.com>
> > Cc: Adrien Mazarguil <adrien.mazarguil@6wind.com>; dev@dpdk.org;
> > Thomas Monjalon <thomas@monjalon.net>; Olga Shern
> > <olgas@mellanox.com>; Matan Azrad <matan@mellanox.com>
> > Subject: Re: [dpdk-dev] [PATCH v2 4/7] net/mlx4: merge Tx path functions
> > 
> > On Tue, Oct 24, 2017 at 08:36:52PM +0000, Ophir Munk wrote:
> > > Hi,
> > >
> > > On Tuesday, October 24, 2017 4:52 PM, Nélio Laranjeiro wrote:
> > > >
> > > > On Mon, Oct 23, 2017 at 02:21:57PM +0000, Ophir Munk wrote:
> > > > > From: Matan Azrad <matan@mellanox.com>
> > > > >
> > > > > Merge tx_burst and mlx4_post_send functions to prevent double
> > > > > asking about WQ remain space.
> > > > >
> > > > > This should improve performance.
> > > > >
> > > > > Signed-off-by: Matan Azrad <matan@mellanox.com>
> > > > > ---
> > > > >  drivers/net/mlx4/mlx4_rxtx.c | 353
> > > > > +++++++++++++++++++++----------------------
> > > > >  1 file changed, 170 insertions(+), 183 deletions(-)
> > > >
> > > > What are the real expectation you have on the remaining patches of
> > > > the series?
> > > >
> > > > According to the comment of this commit log "This should improve
> > > > performance" there are too many barriers at each packet/segment
> > > > level to improve something.
> > > >
> > > > The point is, mlx4_burst_tx() should write all the WQE without any
> > > > barrier as it is processing a burst of packets (whereas Verbs
> > > > functions which may only process a single packet).
> > >
> > > > The lonely barrier which should be present is the one to ensure that
> > > > all the host memory is flushed before triggering the Tx doorbell.
> > > >
> > >
> > > There is a known ConnectX-3 HW limitation: the first 4 bytes of every
> > > TXWBB (64 bytes chunks) should be
> > > written in a reversed order (from last TXWBB to first TXWBB).
> > 
> > This means the first WQE filled by the burst function is the doorbell.
> > In such situation, the first four bytes of it can be written before
> > leaving the burst function and after a write memory barrier.
> > 
> > Until this first WQE is not complete, the NIC won't start processing the
> > packets.  Memory barriers per packets becomes useless.
> 
> I think this is not true, Since mlx4 HW can prefetch advanced TXbbs if their first 4
> bytes are valid in spite of the first WQE is still not valid (please read the spec).

A compiler barrier is enough on x86 to forbid the CPU to re-order the
instructions, on arm you need a memory barrier, there is a macro in DPDK
for that, rte_io_wmb().

Before triggering the doorbell you must flush the case, this is the only
place where the rte_wmb() should be used.

> > It gives something like:
> > 
> >  uint32_t tx_bb_db = 0;
> >  void *first_wqe = NULL;
> > 
> >  /*
> >   * Prepare all Packets by writing the WQEs without the 4 first bytes of
> >   * the first WQE.
> >   */
> >  for () {
> >  	if (!wqe) {
> > 		first_wqe = wqe;
> > 		tx_bb_db = foo;
> > 	}
> >  }
> >  /* Leaving. */
> >  rte_wmb();
> >  *(uin32_t*)wqe = tx_bb_db;
> >  return n;
> >
> 
> I will take care to check if we can do 2 loops:
> Write all  last 60B per TXbb.
> Memory barrier.
> Write all first 4B per TXbbs.
> 
> > > The last 60 bytes of any TXWBB can be written in any order (before
> > > writing the first 4 bytes).
> > > Is your last statement (using lonely barrier) is in accordance with
> > > this limitation? Please explain.
> > >
> > > > There is also too many cases handled which are useless in bursts
> > situation,
> > > > this function needs to be re-written to its minimal use case i.e.
> > processing a
> > > > valid burst of packets/segments and triggering at the end of the burst the
> > Tx
> > > > doorbell.
> > > >
> > 
> > Regards,
> > 
> > --
> > Nélio Laranjeiro
> > 6WIND

Regards,

-- 
Nélio Laranjeiro
6WIND