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Marchesi" , Beau Belgrave , Jens Remus , Andrew Morton , Jens Axboe , Florian Weimer References: <20250701005321.942306427@goodmis.org> <20250701005451.571473750@goodmis.org> <20250702163609.GR1613200@noisy.programming.kicks-ass.net> <20250702124216.4668826a@batman.local.home> <20250702132605.6c79c1ec@batman.local.home> <20250702134850.254cec76@batman.local.home> <482f6b76-6086-47da-a3cf-d57106bdcb39@efficios.com> <20250702150535.7d2596df@batman.local.home> <47a43d27-7eac-4f88-a783-afdd3a97bb11@efficios.com> <20250702152111.1bec7214@batman.local.home> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20250702152111.1bec7214@batman.local.home> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQBPR01CA0040.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:2::12) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YQXPR01MB5930:EE_ X-MS-Office365-Filtering-Correlation-Id: 07af6954-33cd-470c-78ea-08ddb9a0aca2 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|1800799024|366016; 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As soon as the same thread has a new id you can >>> assume all the older user space traces are not applicable for any new >>> events for that thread, or any other thread with the same thread ID. >> >> In order for the scheme you describe to work, you need: >> >> - instrumentation of task lifetime (exit/fork+clone), >> - be sure that the events related to that instrumentation were not >> dropped. >> >> I'm not sure about ftrace, but in LTTng enabling instrumentation of >> task lifetime is entirely up to the user. > > Has nothing to do with task lifetime. If you see a deferred request > with id of 1 from task 8888, and then later you see either a deferred > request or a stack trace with an id other than 1 for task 8888, you can > then say all events before now are no longer eligible for new deferred > stack traces. > >> >> And even if it's enabled, events can be discarded (e.g. buffer full). > > The only case is if you see a deferred request with id 1 for task 8888, > then you start dropping all events and that task 8888 exits and a new > one appears with task id 8888 where it too has a deferred request with > id 1 then you start picking up events again and see a deferred stack > trace for the new task 8888 where it's id is 1, you lose. > > But other than that exact scenario, it should not get confused. Correct. > >> >>> >>> Thus the only issue that can truly be a problem is if you have missed >>> events where thread id wraps around. I guess that could be possible if >>> a long running task finally exits and it's thread id is reused >>> immediately. Is that a common occurrence? >> >> You just need a combination of thread ID re-use and either no >> instrumentation of task lifetime or events discarded to trigger this. > > Again, it's seeing a new request with another id for the same task, you > don't need to worry about it. You don't even need to look at fork and > exit events. The reason why instrumentation of exit/{fork,clone} is useful is to know when a thread ID is re-used. > >> Even if it's not so frequent, at large scale and in production, I >> suspect that this will happen quite often. > > Really? As I explained above? Note that all newly forked threads will likely start counting near 0. So chances are that for short-lived threads most of the counter values will be in a low range. So all you need is thread ID re-use for two threads which happen to use the deferred cookies within low-value ranges to hit this. From my perspective, making trace analysis results reliable is the most basic guarantee tooling should provide in order to make it trusted by users. So I am tempted to err towards robustness rather than take shortcuts because "it does not happen often". Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com