Re: [PATCH net v2] net: mana: reserve RX buffer headroom to fix forwarding performance

[sashiko-bot@kernel.org]

> Commit 730ff06d3f5c ("net: mana: Use page pool fragments for RX buffers
> instead of full pages to improve memory efficiency.") started handing
> out RX buffers with zero headroom so that two buffers fit into one page
> at the default MTU.
> 
> The MANA TX path, however, stores the per scatter-gather entry DMA
> mappings in `struct mana_skb_head` at skb->head, and mana_start_xmit()
> therefore calls skb_cow_head(skb, MANA_HEADROOM). The port advertises
> this requirement as ndev->needed_headroom = MANA_HEADROOM.
> 
> As a result every packet that is received and then forwarded out of a
> MANA port fails the skb_cow() in ip_forward() and gets reallocated and
> copied by pskb_expand_head(). This is invisible to a plain RX or TX
> workload, but it puts a full skb reallocation plus memcpy on the hot
> path of every single forwarded packet, which is exactly what a
> [ ... ]
>   after               0.94%         0.64%         gone
> 
> Cc: stable@vger.kernel.org
> Fixes: 730ff06d3f5c ("net: mana: Use page pool fragments for RX buffers instead of full pages to improve memory efficiency.")
> Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261003013647.2051416-1-hamzamahfooz@linux.microsoft.com?part=1