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Wed, 20 Aug 2025 20:29:32 +0000 Message-ID: <81c1a391-3193-41c6-8ab7-c50c58684a22@intel.com> Date: Wed, 20 Aug 2025 13:29:31 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [Intel-wired-lan] [PATCH iwl-next] igb: Retrieve Tx timestamp directly from interrupt To: Miroslav Lichvar CC: Kurt Kanzenbach , Tony Nguyen , Przemek Kitszel , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , "Paolo Abeni" , Richard Cochran , Vinicius Costa Gomes , Sebastian Andrzej Siewior , , References: <20250815-igb_irq_ts-v1-1-8c6fc0353422@linutronix.de> Content-Language: en-US From: Jacob Keller Autocrypt: addr=jacob.e.keller@intel.com; keydata= xjMEaFx9ShYJKwYBBAHaRw8BAQdAE+TQsi9s60VNWijGeBIKU6hsXLwMt/JY9ni1wnsVd7nN J0phY29iIEtlbGxlciA8amFjb2IuZS5rZWxsZXJAaW50ZWwuY29tPsKTBBMWCgA7FiEEIEBU qdczkFYq7EMeapZdPm8PKOgFAmhcfUoCGwMFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AA CgkQapZdPm8PKOiZAAEA4UV0uM2PhFAw+tlK81gP+fgRqBVYlhmMyroXadv0lH4BAIf4jLxI UPEL4+zzp4ekaw8IyFz+mRMUBaS2l+cpoBUBzjgEaFx9ShIKKwYBBAGXVQEFAQEHQF386lYe MPZBiQHGXwjbBWS5OMBems5rgajcBMKc4W4aAwEIB8J4BBgWCgAgFiEEIEBUqdczkFYq7EMe apZdPm8PKOgFAmhcfUoCGwwACgkQapZdPm8PKOjbUQD+MsPBANqBUiNt+7w0dC73R6UcQzbg cFx4Yvms6cJjeD4BAKf193xbq7W3T7r9BdfTw6HRFYDiHXgkyoc/2Q4/T+8H In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="------------sCqNIkIq5RKPQIw0U3pHwRmm" X-ClientProxiedBy: MW2PR16CA0050.namprd16.prod.outlook.com (2603:10b6:907:1::27) To CO1PR11MB5089.namprd11.prod.outlook.com (2603:10b6:303:9b::16) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO1PR11MB5089:EE_|DM3PPFB266726CF:EE_ X-MS-Office365-Filtering-Correlation-Id: dc5cdba9-a4a5-44a3-44ec-08dde028457e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|376014|366016; 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boundary="------------XqR9oFVG8vzSjuDAeArUTFYY"; protected-headers="v1" From: Jacob Keller To: Miroslav Lichvar Cc: Kurt Kanzenbach , Tony Nguyen , Przemek Kitszel , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Richard Cochran , Vinicius Costa Gomes , Sebastian Andrzej Siewior , intel-wired-lan@lists.osuosl.org, netdev@vger.kernel.org Message-ID: <81c1a391-3193-41c6-8ab7-c50c58684a22@intel.com> Subject: Re: [Intel-wired-lan] [PATCH iwl-next] igb: Retrieve Tx timestamp directly from interrupt References: <20250815-igb_irq_ts-v1-1-8c6fc0353422@linutronix.de> In-Reply-To: --------------XqR9oFVG8vzSjuDAeArUTFYY Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 8/20/2025 12:56 AM, Miroslav Lichvar wrote: > On Tue, Aug 19, 2025 at 04:31:49PM -0700, Jacob Keller wrote: >> I'm having trouble interpreting what exactly this data shows, as its >> quite a lot of data and numbers. I guess that it is showing when it >> switches over to software timestamps.. It would be nice if ntpperf >> showed number of events which were software vs hardware timestamping, = as >> thats likely the culprit. igb hardare only has a single outstanding Tx= >> timestamp at a time. >=20 > The server doesn't have a way to tell the client (ntpperf) which > timestamps are HW or SW, we can only guess from the measured offset as > HW timestamps should be more accurate, but on the server side the > number of SW and HW TX timestamps provided to the client can be > monitored with the "chronyc serverstats" command. The server requests > both SW and HW TX timestamps and uses the better one it gets from the > kernel, if it can actually get one before it receives the next > request from the same client (ntpperf simulates up to 16384 concurrent > clients). >=20 > When I run ntpperf at a fixed rate of 140000 requests per second > for 10 seconds (-r 140000 -t 10), I get the following numbers. >=20 > Without the patch: > NTP daemon TX timestamps : 28056 > NTP kernel TX timestamps : 1012864 > NTP hardware TX timestamps : 387239 >=20 > With the patch: > NTP daemon TX timestamps : 28047 > NTP kernel TX timestamps : 707674 > NTP hardware TX timestamps : 692326 >=20 > The number of HW timestamps is significantly higher with the patch, so > that looks good. >=20 > But when I increase the rate to 200000, I get this: >=20 > Without the patch: > NTP daemon TX timestamps : 35835 > NTP kernel TX timestamps : 1410956 > NTP hardware TX timestamps : 581575 =20 >=20 > With the patch: > NTP daemon TX timestamps : 476908 > NTP kernel TX timestamps : 646146 > NTP hardware TX timestamps : 412095 >=20 When does the NTP daemon decide to go with timestamping within the daemon vs timestamping in the kernel? It seems odd that we don't achieve 100% kernel timestamps... > With the patch, the server is now dropping requests and can provide > a smaller number of HW timestamps and also a smaller number of SW > timestamps, i.e. less work is done overall. >=20 > Could the explanation be that a single CPU core now needs to do more > work, while it was better distributed before? >=20 Hm. The interrupt vector may be fired on the same CPU maybe? The work items can go into the general pool which spreads to all CPUs, and I guess the amount of work to submit the timestamp is high enough that we do end up costing too much? Hmm. We could experiment with using a kthread via the ptp_aux_work setup and tuning to ensure that thread has good prioritization? I don't know what the best compromise is since its clear the interrupt is best if the timestamp volume isn't too high. --------------XqR9oFVG8vzSjuDAeArUTFYY-- --------------sCqNIkIq5RKPQIw0U3pHwRmm Content-Type: application/pgp-signature; name="OpenPGP_signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="OpenPGP_signature.asc" -----BEGIN PGP SIGNATURE----- wnsEABYIACMWIQQgQFSp1zOQVirsQx5qll0+bw8o6AUCaKYwKwUDAAAAAAAKCRBqll0+bw8o6GkL APoCmOpmt/7leIyVzz6A8bNd9z5mNJHy7laW9hSSGPCjnQD8CtgaH4SAdqvw18VHcTmkcpBkCiVX UsLzW6Iw0cjKrAM= =ECEP -----END PGP SIGNATURE----- --------------sCqNIkIq5RKPQIw0U3pHwRmm--