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

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Message-ID: <8e7f3798-c303-44b9-ae3f-5343f7f811e8@linux.ibm.com>
Date: Fri, 6 Dec 2024 16:20:50 +0100
From: Alexandra Winter <wintera@...ux.ibm.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Rahul Rameshbabu <rrameshbabu@...dia.com>,
        Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
        Leon Romanovsky <leon@...nel.org>, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Andrew Lunn <andrew+netdev@...n.ch>,
        Nils Hoppmann <niho@...ux.ibm.com>, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev
 <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Thorsten Winkler <twinkler@...ux.ibm.com>,
        Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next] net/mlx5e: Transmit small messages in linear skb



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?