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client-ip=165.204.84.17; helo=satlexmb07.amd.com; pr=C Received: from satlexmb07.amd.com (165.204.84.17) by SJ1PEPF000026C6.mail.protection.outlook.com (10.167.244.103) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.25.13 via Frontend Transport; Wed, 13 May 2026 06:57:42 +0000 Received: from satlexmb10.amd.com (10.181.42.219) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.41; Wed, 13 May 2026 01:57:41 -0500 Received: from satlexmb07.amd.com (10.181.42.216) by satlexmb10.amd.com (10.181.42.219) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.41; Wed, 13 May 2026 01:57:41 -0500 Received: from [172.31.184.125] (10.180.168.240) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server id 15.2.2562.41 via Frontend Transport; Wed, 13 May 2026 01:57:38 -0500 Message-ID: <35f10be6-1e2f-4a6f-ae9f-a09754221011@amd.com> Date: Wed, 13 May 2026 12:27:37 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/3] sched/fair: Add compile time check in fastpaths for CONFIG_SCHED_SMT=n To: Shrikanth Hegde , , , , CC: , , , , , , , References: <20260512152125.308280-1-sshegde@linux.ibm.com> <20260512152125.308280-4-sshegde@linux.ibm.com> <0b3d8b79-6743-4f35-abc8-da4f2a6a2d80@linux.ibm.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <0b3d8b79-6743-4f35-abc8-da4f2a6a2d80@linux.ibm.com> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: IQ0l1Lm1UA0lmesrPfR7+NSbs3hRJaDyO9K23+TgeSDEENl7PEMWvG71FFe6rUxjiew+N9DM4cR/vXq2t+9VMAK7lmcr0TiZN0Hlh4yYUSSiWoresPRZcvamJ1Ur/TwNRCcWslAmwjSJhw1CfzPtRpF5CVdDT65Smoo0YemKI1O+a+n7+b6sVz6w/Aaq7mTyXD9Rc3CYJc7e2IUFVNv3DzAJfLbaD6zQPhbLwC2JC5CXV0RRhT7/xVCITFmmqIVTNg0l1PKJGm/oD7itpImhalcHkyUL6MEeW5hpzWc5kPSHzeAyUOl7+9hD+jexDyQmQDRjWmdfEZfNc+NnEuT9WCIZy2KyqPaiuRnRFlCkpdE4JjzE4VEWWKzFlzHOmHgz4dWqoAMAmZ9aFAKqMvEFgiZufoHTLIVNkhrREiJyOMc3TCy1ZSzEuAM5x5gpo27F X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 13 May 2026 06:57:42.4542 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: edae55e6-2b81-4cee-818f-08deb0bcedf6 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=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ1PEPF000026C6.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY1PR12MB9651 Hello Shrikanth, On 5/13/2026 12:09 PM, Shrikanth Hegde wrote: > > Hi Prateek. > >> ... I think the if() above can simply check for sched_smt_active() >> before calling is_core_idle() like: >> >>      if (sched_smt_active() && >>          !(env->sd->flags & SD_SHARE_CPUCAPACITY) && >>          !is_core_idle(cpu)) { >>          ... >>      } >> >> That way, we ensure we don't call is_core_idle() and the cpumask_and() >> unnecessarily on topologies that don't have SMT domain similar to how >> sched_use_asym_prio() guards the call to is_core_idle(). >> >> Thoughts? >> > In addition, Do you think it makes sense to consolidate use of > sched_smt_present always using sched_smt_active() ? > > > static __always_inline bool sched_smt_active(void) > { >         return static_branch_likely(&sched_smt_present); > } > > kernel/sched/core_sched.c:      if (!static_branch_likely(&sched_smt_present)) > kernel/sched/fair.c:    if (!static_branch_likely(&sched_smt_present) || > kernel/sched/sched.h:   if (static_branch_unlikely(&sched_smt_present)) > > > The last being differing with unlikely in update_idle_core. > Does it make sense to call sched_smt_active there as well? I think using sched_smt_active() everywhere should be fine. I'm not sure what made update_idle_core() special; Perhaps there was a greater chance of running it on systems with CONFIG_SCHED_SMT enabled but doesn't physically have SMT. This came from commit 1b568f0aabf2 ("sched/core: Optimize SCHED_SMT") in 2016 when SMT was perhaps not so common and sched_smt_active() came later in 2018 with the spectre mitigation optimizations on x86 where SMT was more prevalent so it defaulted to likely. I'll let Peter comment if he prefers keeping this branch unlikely but from my side, unifying eveything under sched_smt_active() looks better. -- Thanks and Regards, Prateek