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Tue, 10 Dec 2024 13:55:13 +0000 Message-ID: Date: Tue, 10 Dec 2024 14:54:26 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next] net/mlx5e: Transmit small messages in linear skb To: Dragos Tatulea CC: Alexandra Winter , Rahul Rameshbabu , Saeed Mahameed , Tariq Toukan , Leon Romanovsky , David Miller , Jakub Kicinski , Paolo Abeni , Eric Dumazet , Andrew Lunn , Nils Hoppmann , , , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Thorsten Winkler , Simon Horman References: <20241204140230.23858-1-wintera@linux.ibm.com> <8e7f3798-c303-44b9-ae3f-5343f7f811e8@linux.ibm.com> <554a3061-5f3b-4a7e-a9bd-574f2469f96e@nvidia.com> From: Alexander Lobakin Content-Language: en-US In-Reply-To: <554a3061-5f3b-4a7e-a9bd-574f2469f96e@nvidia.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MI1P293CA0018.ITAP293.PROD.OUTLOOK.COM (2603:10a6:290:3::12) To DS0PR11MB8718.namprd11.prod.outlook.com (2603:10b6:8:1b9::20) Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB8718:EE_|SA1PR11MB5801:EE_ X-MS-Office365-Filtering-Correlation-Id: 4ea847ca-d70c-4c3e-776f-08dd192244f8 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014|7053199007; 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>>>> >>>> + /* 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. On different setups, usually the copybreak of 192 or 256 bytes was the most efficient as well. > > Thanks, > Dragos Thanks, Olek