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).
