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Mon, 19 Jan 2026 12:11:50 +0000 Message-ID: Date: Mon, 19 Jan 2026 14:11:43 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] perf cs-etm: Fix decoding for sparse CPU maps To: Leo Yan , James Clark CC: Suzuki K Poulose , Mike Leach , John Garry , Will Deacon , Leo Yan , Peter Zijlstra , Ingo Molnar , "Arnaldo Carvalho de Melo" , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , "Ian Rogers" , Thomas Falcon , , , , References: <20260119-james-perf-coresight-cpu-map-segfault-v2-0-56b956a629ee@linaro.org> <20260119-james-perf-coresight-cpu-map-segfault-v2-1-56b956a629ee@linaro.org> <20260119111509.GD1286628@e132581.arm.com> Content-Language: en-US From: Adrian Hunter Organization: Intel Finland Oy, Registered Address: c/o Alberga Business Park, 6 krs, Bertel Jungin Aukio 5, 02600 Espoo, Business Identity Code: 0357606 - 4, Domiciled in Helsinki In-Reply-To: <20260119111509.GD1286628@e132581.arm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: DU2PR04CA0054.eurprd04.prod.outlook.com (2603:10a6:10:234::29) To IA1PR11MB7198.namprd11.prod.outlook.com (2603:10b6:208:419::15) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA1PR11MB7198:EE_|SA2PR11MB4969:EE_ X-MS-Office365-Filtering-Correlation-Id: 8278c714-b0bf-4ce5-6be7-08de5753ecd9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; 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This assumption is used for inserting records >> into the queue, and for fetching queues when given a CPU number. This >> assumption held when Perf always opened a dummy event on every CPU, even >> if the user provided a subset of CPUs on the commandline, resulting in >> the indices aligning. >> >> For example: >> >> # event : name = cs_etm//u, , id = { 2451, 2452 }, type = 11 (cs_etm), size = 136, config = 0x4010, { sample_period, samp> >> # event : name = dummy:u, , id = { 2453, 2454, 2455, 2456 }, type = 1 (PERF_TYPE_SOFTWARE), size = 136, config = 0x9 (PER> >> >> 0 0 0x200 [0xd0]: PERF_RECORD_ID_INDEX nr: 6 >> ... id: 2451 idx: 2 cpu: 2 tid: -1 >> ... id: 2452 idx: 3 cpu: 3 tid: -1 >> ... id: 2453 idx: 0 cpu: 0 tid: -1 >> ... id: 2454 idx: 1 cpu: 1 tid: -1 >> ... id: 2455 idx: 2 cpu: 2 tid: -1 >> ... id: 2456 idx: 3 cpu: 3 tid: -1 >> >> Since commit 811082e4b668 ("perf parse-events: Support user CPUs mixed >> with threads/processes") the dummy event no longer behaves in this way, >> making the ETM event indices start from 0 on the first CPU recorded >> regardless of its ID: >> >> # event : name = cs_etm//u, , id = { 771, 772 }, type = 11 (cs_etm), size = 144, config = 0x4010, { sample_period, sample> >> # event : name = dummy:u, , id = { 773, 774 }, type = 1 (PERF_TYPE_SOFTWARE), size = 144, config = 0x9 (PERF_COUNT_SW_DUM> >> >> 0 0 0x200 [0x90]: PERF_RECORD_ID_INDEX nr: 4 >> ... id: 771 idx: 0 cpu: 2 tid: -1 >> ... id: 772 idx: 1 cpu: 3 tid: -1 >> ... id: 773 idx: 0 cpu: 2 tid: -1 >> ... id: 774 idx: 1 cpu: 3 tid: -1 > > Seems to me that this patch works around the issue by using the CPU ID > instead, but event->auxtrace.idx is broken. > > Should we store the correct index in event->auxtrace.idx (e.g., in the > __perf_event__synthesize_id_index()) ? The idx value represents a perf events ring buffer. Events on the same CPU can share the same ring buffer. But in the case of per-thread recording, different threads have different ring buffers and therefore different idx values. So I don't think the idx value is wrong. It is just not the same thing as CPU number.