Re: [PATCH net-next] net/mlx5e: Transmit small messages in linear skb

[Joe Damato <jdamato@fastly.com>]

On Tue, Dec 10, 2024 at 02:54:26PM +0100, Alexander Lobakin wrote:
> From: Dragos Tatulea <dtatulea@nvidia.com>
> Date: Tue, 10 Dec 2024 12:44:04 +0100
> 
> > 
> > 
> > On 06.12.24 16:20, Alexandra Winter wrote:
> >>
> >>
> >> On 04.12.24 15:32, Alexander Lobakin wrote:
> >>>> @@ -269,6 +270,10 @@ static void mlx5e_sq_xmit_prepare(struct mlx5e_txqsq *sq, struct sk_buff *skb,
> >>>>  {
> >>>>  	struct mlx5e_sq_stats *stats = sq->stats;
> >>>>  
> >>>> +	/* Don't require 2 IOMMU TLB entries, if one is sufficient */
> >>>> +	if (use_dma_iommu(sq->pdev) && skb->truesize <= PAGE_SIZE)
> >>    +		skb_linearize(skb);
> >>> 1. What's with the direct DMA? I believe it would benefit, too?
> >>
> >>
> >> Removing the use_dma_iommu check is fine with us (s390). It is just a proposal to reduce the impact.
> >> Any opinions from the NVidia people?
> >>
> > Agreed.
> > 
> >>
> >>> 2. Why truesize, not something like
> >>>
> >>> 	if (skb->len <= some_sane_value_maybe_1k)
> >>
> >>
> >> With (skb->truesize <= PAGE_SIZE) the whole "head" buffer fits into 1 page.
> >> When we set the threshhold at a smaller value, skb->len makes more sense
> >>
> >>
> >>>
> >>> 3. As Eric mentioned, PAGE_SIZE can be up to 256 Kb, I don't think
> >>>    it's a good idea to rely on this.
> >>>    Some test-based hardcode would be enough (i.e. threshold on which
> >>>    DMA mapping starts performing better).
> >>
> >>
> >> A threshhold of 4k is absolutely fine with us (s390). 
> >> A threshhold of 1k would definitvely improve our situation and bring back the performance for some important scenarios.
> >>
> >>
> >> NVidia people do you have any opinion on a good threshhold?
> >>
> > 1KB is still to large. As Tariq mentioned, the threshold should not
> > exceed 128/256B. I am currently testing this with 256B on x86. So far no
> > regressions but I need to play with it more.
> 
> On different setups, usually the copybreak of 192 or 256 bytes was the
> most efficient as well.

A minor suggestion:

Would it be at all possible for the people who've run these
experiments to document their findings somewhere: what the different
test setups were, what the copybreak settings were, what the
results were, and how they were measured?

Some drivers have a few details documented in
Documentation/networking/device_drivers/ethernet/, but if others
could do this too, like mlx5, in detail so findings could be
reproduced by others, that would be amazing.