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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <554a3061-5f3b-4a7e-a9bd-574f2469f96e@nvidia.com>
Date: Tue, 10 Dec 2024 12:44:04 +0100
From: Dragos Tatulea <dtatulea@...dia.com>
To: Alexandra Winter <wintera@...ux.ibm.com>,
 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 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