Re: [PATCH v3 1/7] net/mlx4: remove error flows from Tx fast path

[Matan Azrad <matan@mellanox.com>]

Hi Adrien

> -----Original Message-----
> From: Adrien Mazarguil [mailto:adrien.mazarguil@6wind.com]
> Sent: Monday, October 30, 2017 4:23 PM
> To: Matan Azrad <matan@mellanox.com>
> Cc: dev@dpdk.org; Ophir Munk <ophirmu@mellanox.com>
> Subject: Re: [PATCH v3 1/7] net/mlx4: remove error flows from Tx fast path
> 
> On Mon, Oct 30, 2017 at 10:07:23AM +0000, Matan Azrad wrote:
> > Move unnecessary error flows to DEBUG mode.
> >
> > Signed-off-by: Matan Azrad <matan@mellanox.com>
> > Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> 
> I missed a couple of details while reviewing the original version, the first one
> being mlx4_post_send()'s return value is still documented as updating
> rte_errno in case of error, it's not the case anymore after this patch.
> 
Good attention, Will be fixed in next version.

> Please see below for the other one:
> 
> > ---
> >  drivers/net/mlx4/mlx4_rxtx.c | 16 ++++++----------
> >  1 file changed, 6 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/net/mlx4/mlx4_rxtx.c
> > b/drivers/net/mlx4/mlx4_rxtx.c
> <snip>
> >  /**
> > @@ -510,8 +508,6 @@ struct pv {
> >  	assert(max <= elts_n);
> >  	/* Always leave one free entry in the ring. */
> >  	--max;
> > -	if (max == 0)
> > -		return 0;
> >  	if (max > pkts_n)
> >  		max = pkts_n;
> >  	for (i = 0; (i != max); ++i) {
> 
> While minor, this change has nothing to do with this patch, right?
> 
Yes you right, maybe it can be merged in patch 4/7.
 
> I think it can slightly degrade an application performance as it removes the
> guarantee that subsequent code only needs to be run if there is at least one
> packet to process in case the TX ring is constantly full (SW faster than HW).
>

In case the TX ring is full, the loop condition should fail in the start and then return with 0  because the packet counter is 0.(more 2 checks)
Since this case are less common (in my opinion) than at least 1 free space in ring, we can prevent this unnecessary check for all these common cases.    
  
Are you sure the 2 extra check important for performance in this empty case? Doesn't the application will call us again? 
 
> Can you remove it?
> 
> --
> Adrien Mazarguil
> 6WIND