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Received-SPF: Pass (protection.outlook.com: domain of amd.com designates 165.204.84.17 as permitted sender) receiver=protection.outlook.com; client-ip=165.204.84.17; helo=satlexmb08.amd.com; pr=C Received: from satlexmb08.amd.com (165.204.84.17) by SJ1PEPF000023CC.mail.protection.outlook.com (10.167.244.6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.615.4 via Frontend Transport; Sat, 14 Nov 2026 03:48:23 +0000 Received: from satlexmb08.amd.com (10.181.42.217) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.53; Fri, 13 Nov 2026 21:48:22 -0600 Received: from [192.168.0.101] (10.180.168.240) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server id 15.2.2562.53 via Frontend Transport; Fri, 13 Nov 2026 21:48:17 -0600 Message-ID: <0f97689d-fdec-4cf4-b6ca-4365aced16d6@amd.com> Date: Sat, 14 Nov 2026 09:18:16 +0530 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] perf/x86/amd/lbr: Freeze LBR on PMI only when kernel branches are recorded To: Puranjay Mohan , , CC: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , "Alexander Shishkin" , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , Thomas Gleixner , Borislav Petkov , Dave Hansen , , "H. Peter Anvin" , Andrii Nakryiko , Ravi Bangoria References: <20261023124157.75524-1-puranjay@kernel.org> Content-Language: en-US From: Ravi Bangoria In-Reply-To: <20261023124157.75524-1-puranjay@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ1PEPF000023CC:EE_|IA5PR12MB999276:EE_ X-MS-Office365-Filtering-Correlation-Id: 60beb6c8-278f-4b57-28a0-08df420221ac X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|36860700016|23010399003|376014|7416014|82310400026|1800799024|11063799006|56012099006|261021123027099003|261021121427099003|261026121527099003|261009223027099003|261026140227099003|261022105027099003|261102123227099003|261028144727099003|22082099003|18002099003|10067099003; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:165.204.84.17;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:satlexmb08.amd.com;PTR:InfoDomainNonexistent;CAT:NONE;SFS:(13230040)(36860700016)(23010399003)(376014)(7416014)(82310400026)(1800799024)(11063799006)(56012099006)(261021123027099003)(261021121427099003)(261026121527099003)(261009223027099003)(261026140227099003)(261022105027099003)(261102123227099003)(261028144727099003)(22082099003)(18002099003)(10067099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: iWkiC8L0UBocgWqQeW7XA0iRNgSK48fiSlMNm3ByAm4h9F/BgcE/ZgY8vNqZtLjiBF0E4pO7exK5iw+cx0R9hyLir2epp1Z665xQh93bgBvyB9H95EAFt/t7uinBtN4POP5kIxpB5P+lGhy0o6iyexLqmxLImeH+aN3V6ZgTn6YBWBZpHYszg0wUXGu4VqmKj+S1atw8DAPiTlrgP/a+RKPVDxMFhe8kIO11UFggdIrbX99e5vZVf9jDB8M+24S+c0az5r9NIANh8fwHXH9eWcH1FHW0mIM+9pf0rbue5ykLWwgkhqIBkLS1/1/YDKiPPK4ZcUbvCg+VoPFzbGGt72tpHsYzetYYw7cZdU4gOmNme1bnT1S71mlw+v6wqHp8PoqYXXDScG9d5EJPKRkFElhLD9z5h8ZqcSdyTFFKxqdGrPhQZ+Rf+HIspVLAKkyn X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 14 Nov 2026 03:48:23.0937 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 60beb6c8-278f-4b57-28a0-08df420221ac X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb08.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ1PEPF000023CC.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA5PR12MB999276 On 23-Oct-26 6:11 PM, Puranjay Mohan wrote: > On CPUs with X86_FEATURE_AMD_LBR_PMC_FREEZE the LBR is always frozen on > PMI, so the branch stack ends before the IP and callchain, which are > taken when the NMI is delivered. IBS rejects branch-stack events, so > there is no precise alternative. > > The freeze only keeps the PMI handler's branches out of the stack, which > matters only when branches into the kernel are recorded; without it the > stack fills with the handler's own branches. A user-only filter does not > record them, and parts without the feature are never frozen (see commit > 598c2fafc06f ("perf/x86/amd/lbr: Use freeze based on availability")). > > Freeze unless the value written to LBR_SELECT, which is shared by all > events on the CPU, suppresses branches into the kernel. > > With perf record -e ex_ret_brn_tkn:u -j any,u on a Zen 5 part running > CPython, the function of the sampled IP differs from the function of > LBR[0].to in 84.3% of samples with the freeze and in 0.1% without it. > > Signed-off-by: Puranjay Mohan Reviewed-by: Ravi Bangoria Thanks, Ravi