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Marchesi" , Beau Belgrave , Jens Remus , Linus Torvalds , Andrew Morton , tech-board-discuss@lists.linuxfoundation.org References: <20250709212556.32777-1-mathieu.desnoyers@efficios.com> <20250709212556.32777-3-mathieu.desnoyers@efficios.com> <20250711065742.00d6668b@gandalf.local.home> <2025071124-cola-slouching-9dd2@gregkh> <20250711100239.55434108@gandalf.local.home> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20250711100239.55434108@gandalf.local.home> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQBPR0101CA0266.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:68::31) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YQBPR01MB10877:EE_ X-MS-Office365-Filtering-Correlation-Id: 3dcc8a77-64ee-497e-4e3d-08ddc085b180 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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This is >> one of those penalities, sorry. > > BTW, when it comes to tracers, being out of tree is actually a huge > advantage. Tracers, unlike drivers, are only monitoring the kernel. Which > means it doesn't really rely on the workings of the kernel, so it really > doesn't get much help from changes made by other kernel developers. Like > the BPF folks have said keeping BPF programs up to date, isn't that much of > a burden. > > The huge advantage that LTTng has over perf and ftrace for being out of > tree is that it controls the ABI between the LTTng kernel side and the user > space side. LTTng can experiment with new interfaces, and if something > breaks, it can simply change it and deliver a new tool that includes the > new module with the update. > > One thing that perf and ftrace struggle with is getting the API correct the > first time. That's because they have no control over what other tools may use > their interface. If I expose something that another tool starts to use > (like powertop) and then I change it to enhance tracing and it breaks that > tool, that change gets reverted. To make any new update, I have to guess at > what are all the ways a tool could abuse this interface and also make sure > it works well into the future when new things come up. There's a lot of > interfaces I would love to change but can't, simply because they are now > ABI. > > LTTng doesn't have the burden of worrying about "what other tool may use > this kernel interface?", and can even change it much later if Mathieu finds > a better way to do it. This is indeed one advantage LTTng has: it controls both the kernel and userspace sides of the software. I should point out however that since about LTTng 2.7 (2015) we've made it a project policy to never break the LTTng-modules userspace ABI. Those ABIs are effectively append-only (with possible gradual deprecation) to make life easier for distro vendors for which the userspace vs kernel modules packages are not in sync sometimes. ABI-breaking changes to LTTng-modules makes packaging of kernel + OOT modules + userspace tricky for distros. Similarly to the Linux kernel, our "do not break userspace ABI" policy takes care of this. So even though LTTng does not _break_ ABIs, it benefits from controlling what userspace is allowed to interact with it. The fact that we have an aligned release cycle across LTTng-Tools, the LTTng userspace tracer, and the LTTng kernel tracer allows us to deprecate ABIs at a faster pace than what typical kernel ABIs with arbitrary userspace consumers can do. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com