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Shenoy" , K Prateek Nayak Subject: [PATCH v3 6/8] sched/fair: Remove superfluous rcu_read_lock() in the wakeup path Date: Tue, 20 Jan 2026 11:32:44 +0000 Message-ID: <20260120113246.27987-7-kprateek.nayak@amd.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260120113246.27987-1-kprateek.nayak@amd.com> References: <20260120113246.27987-1-kprateek.nayak@amd.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: satlexmb08.amd.com (10.181.42.217) To satlexmb07.amd.com (10.181.42.216) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BL02EPF00021F6F:EE_|CH3PR12MB8581:EE_ X-MS-Office365-Filtering-Correlation-Id: f03cc2b2-14f1-4968-cd13-08de5817f02a X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|36860700013|376014|7416014|1800799024|82310400026; X-Microsoft-Antispam-Message-Info: =?us-ascii?Q?TXOnKHC3zfopdeCIh/Qyfv2Ecsd7qms7mdLGlPdb9TVWthbzEuCAXDrajUf4?= =?us-ascii?Q?+YIeMDxmaEPDJsxioFroWLPtbhDhTR3zpBwTWhN3KbRVZyKhoBaVV1MLc0MN?= =?us-ascii?Q?jjTFXm4nBGeaR8gpiMU8u7FFYFb1kXDfOUeSDKKE873KWrBlyzNu19MaCP0q?= =?us-ascii?Q?zEw7aTSCNGIV1F0vW4PxVj2LHeD7AFT54U29+QxLStjkX3vih+xdy6AWpZHs?= =?us-ascii?Q?SIa0zrWpnEkk4OHUu/ov7EuwDdPyibC/oxZblco308TOwj3UFP3aBRjqnJpg?= =?us-ascii?Q?tmpCNRbFLndmuUtwIHojqXveTx43m85h1FQ6yYXLJjRO1hC3PE9swZ57cg+N?= =?us-ascii?Q?24kTCyrssmr33P+f+piolTbWrCApe33OwJ64g6S92sh3oIM89k3wcqXwhdNV?= =?us-ascii?Q?nvRzzZBYLyBt1YazqPdnMJA5NsKobPm8RgNKrau2cRZn1J6DHxrGNEkUtjMF?= =?us-ascii?Q?TmrOcm4tNf1vbeElr1ChwA2Tguxln9ceswIcryXuhr682f+7KWRaYonJsuK9?= =?us-ascii?Q?gh6lOhOBfTvUSeFnh9+G0FjCV/pVbfyLjFRF6DXBuJ0HOXeuxoRDiNZgX57l?= =?us-ascii?Q?Oxux7v0UwLaaDPboTNdUYHparvvJapL9RCEDFnct3BsFzP4e3Eqf8serfoNC?= =?us-ascii?Q?J2aQBCekqxpbcdv98XgPoMJHrLb6vycUrkFS1rI3gcuAFNkPXTqVvez+nHTC?= =?us-ascii?Q?EkdbEnneGUn0OQy3UiOKBCmw4UOHRk0I+Yaike9++fVmauXKoru62rRn5MZi?= =?us-ascii?Q?wNK3PXdCoYhA5R8lkbNycINRassADC59nvU1vmp8ai8nXpUboYk80x6ziNtQ?= =?us-ascii?Q?CLkRgSIEHxCZE2fAjPfxXiBfz+aq75zRJz0o3v/XDEc3F2B44I5dsf/aO+by?= =?us-ascii?Q?Eo6a2iQpA1JIzRywGkEkqhpzrdL9Zz8We+o6vvvuXF1S12Jkpqbr68YZFtZ+?= =?us-ascii?Q?WIKK51b5rXsbAXzEu3mc2INLBwYZXm+Ue36j0+TKMieAGOqylmiim3jpfYdt?= =?us-ascii?Q?hQlRN3T4gqsOz6LK99+ZNHMD80xzceuszbTSpEAOm8FmwOuzOeY2uGePN+xR?= =?us-ascii?Q?eyD3bVy1+L7FWQnORNXegjftPEdlhQ/zpMzIt0wMdtpLTGPVixewqy3Zz26C?= =?us-ascii?Q?ZSrywtja2k6o6I2SwX5Zp/28wEfyW9YyPSSv7w5VyWIdVOBbaXTJgzx2bjbm?= =?us-ascii?Q?vQmilwL1telhsQxwEMv9FHuEbVqBURN2q2YONhf0TMdeiKhuJuvZ+KsI1Z91?= =?us-ascii?Q?4pcNEZq4gnLZCAQwaeCS0VknpGxEQlBVgwXOziF5rJKMnt3x/HbnGjTdzuHY?= =?us-ascii?Q?t28Wbuz3VLEBFbB4ptmV4WW74Gl7MUnTgldT8uAUBxTBWOtn93vwdsRzRC37?= =?us-ascii?Q?x8AZMw3tCR9Zc8pjs1Sc0yUSQfvKLDZiq2yz3aG+Xrze7K7CbCUDFfXJFt1C?= =?us-ascii?Q?sPPcnYL2WtHPPVzbrKumXB9cbrapcjkquiQx3jPsP51bqzfaQe6ZGyBe0i2h?= =?us-ascii?Q?BqqZb5C27udYy7AjG5k3YHHe6/2owWxNhitQA/VaEhKds+ovzP9RrD4+gdxV?= =?us-ascii?Q?cETXuzCtN2IHpXQOGNGXbtigOhr1Hi/fVv1so+IPzkgU7pJ0wCOdrbMTCr3E?= =?us-ascii?Q?rIFqaxLlgWXn7nd7rW43y643YnRMjjOczcvmFR8yTJNupi4tOv0l5f32UKRq?= =?us-ascii?Q?5NyWbw=3D=3D?= X-Forefront-Antispam-Report: CIP:165.204.84.17;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:satlexmb07.amd.com;PTR:InfoDomainNonexistent;CAT:NONE;SFS:(13230040)(36860700013)(376014)(7416014)(1800799024)(82310400026);DIR:OUT;SFP:1101; X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Jan 2026 11:34:56.9595 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: f03cc2b2-14f1-4968-cd13-08de5817f02a 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: BL02EPF00021F6F.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR12MB8581 Content-Type: text/plain; charset="utf-8" select_task_rq_fair() is always called with p->pi_lock held and IRQs disabled which makes it equivalent of an RCU read-side. Since commit 71fedc41c23b ("sched/fair: Switch to rcu_dereference_all()") switched to using rcu_dereference_all() in the wakeup path, drop the explicit rcu_read_{lock,unlock}() in the fair task's wakeup path. Future plans to reuse select_task_rq_fair() / find_energy_efficient_cpu() in the fair class' balance callback will do so with IRQs disabled and will comply with the requirements of rcu_dereference_all() which makes this safe keeping in mind future development plans too. Signed-off-by: K Prateek Nayak --- Changelog rfc v2..v3: o New patch. --- kernel/sched/fair.c | 33 ++++++++++++--------------------- 1 file changed, 12 insertions(+), 21 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 04993c763a06..e4f208c44916 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8323,10 +8323,9 @@ static int find_energy_efficient_cpu(struct task_str= uct *p, int prev_cpu) struct perf_domain *pd; struct energy_env eenv; =20 - rcu_read_lock(); pd =3D rcu_dereference_all(rd->pd); if (!pd) - goto unlock; + return target; =20 /* * Energy-aware wake-up happens on the lowest sched_domain starting @@ -8336,13 +8335,13 @@ static int find_energy_efficient_cpu(struct task_st= ruct *p, int prev_cpu) while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd))) sd =3D sd->parent; if (!sd) - goto unlock; + return target; =20 target =3D prev_cpu; =20 sync_entity_load_avg(&p->se); if (!task_util_est(p) && p_util_min =3D=3D 0) - goto unlock; + return target; =20 eenv_task_busy_time(&eenv, p, prev_cpu); =20 @@ -8437,7 +8436,7 @@ static int find_energy_efficient_cpu(struct task_stru= ct *p, int prev_cpu) prev_cpu); /* CPU utilization has changed */ if (prev_delta < base_energy) - goto unlock; + return target; prev_delta -=3D base_energy; prev_actual_cap =3D cpu_actual_cap; best_delta =3D min(best_delta, prev_delta); @@ -8461,7 +8460,7 @@ static int find_energy_efficient_cpu(struct task_stru= ct *p, int prev_cpu) max_spare_cap_cpu); /* CPU utilization has changed */ if (cur_delta < base_energy) - goto unlock; + return target; cur_delta -=3D base_energy; =20 /* @@ -8478,7 +8477,6 @@ static int find_energy_efficient_cpu(struct task_stru= ct *p, int prev_cpu) best_actual_cap =3D cpu_actual_cap; } } - rcu_read_unlock(); =20 if ((best_fits > prev_fits) || ((best_fits > 0) && (best_delta < prev_delta)) || @@ -8486,11 +8484,6 @@ static int find_energy_efficient_cpu(struct task_str= uct *p, int prev_cpu) target =3D best_energy_cpu; =20 return target; - -unlock: - rcu_read_unlock(); - - return target; } =20 /* @@ -8535,7 +8528,6 @@ select_task_rq_fair(struct task_struct *p, int prev_c= pu, int wake_flags) want_affine =3D !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr); } =20 - rcu_read_lock(); for_each_domain(cpu, tmp) { /* * If both 'cpu' and 'prev_cpu' are part of this domain, @@ -8561,14 +8553,13 @@ select_task_rq_fair(struct task_struct *p, int prev= _cpu, int wake_flags) break; } =20 - if (unlikely(sd)) { - /* Slow path */ - new_cpu =3D sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag); - } else if (wake_flags & WF_TTWU) { /* XXX always ? */ - /* Fast path */ - new_cpu =3D select_idle_sibling(p, prev_cpu, new_cpu); - } - rcu_read_unlock(); + /* Slow path */ + if (unlikely(sd)) + return sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag); + + /* Fast path */ + if (wake_flags & WF_TTWU) + return select_idle_sibling(p, prev_cpu, new_cpu); =20 return new_cpu; } --=20 2.34.1