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

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 06/12/2024 17:20, 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?
> 
> 
>> 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?
> 

Hi,

Many approaches in the past few years are going the opposite direction, 
trying to avoid copies ("zero-copy").

In many cases, copy up to PAGE_SIZE means copy everything.
For high NIC speeds this is not realistic.

Anyway, based on past experience, threshold should not exceed "max 
header size" (128/256b).