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.
Thanks,
Dragos
