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Shenoy" , K Prateek Nayak Subject: [PATCH v4 9/9] sched/fair: Simplify SIS_UTIL handling in select_idle_cpu() Date: Thu, 12 Mar 2026 04:44:34 +0000 Message-ID: <20260312044434.1974-10-kprateek.nayak@amd.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260312044434.1974-1-kprateek.nayak@amd.com> References: <20260312044434.1974-1-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: satlexmb07.amd.com (10.181.42.216) To satlexmb07.amd.com (10.181.42.216) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MWH0EPF000C6184:EE_|SA0PR12MB4350:EE_ X-MS-Office365-Filtering-Correlation-Id: 3e111d46-194b-486b-1253-08de7ff276e4 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|36860700016|82310400026|56012099003|22082099003|18002099003; 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:satlexmb07.amd.com;PTR:InfoDomainNonexistent;CAT:NONE;SFS:(13230040)(7416014)(376014)(1800799024)(36860700016)(82310400026)(56012099003)(22082099003)(18002099003);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: WcrsHLlBNF/k2kN92Op4X4kzpjIhaHaPaU2fM/J3lAfUshK4sHePDF8/dWWVCnSIKffchQj9+mE8K0oPVU+kwsW5Hesmp7bxNwmBe97z7Uino0tzci9nmKJSCJpbrKrJHrJCh6fquTmUVJMj4XvAo938juNqlImv4AdY1LKd9S9tuoCFPp+7mVQ5//O0YwOTrGUOA/FuiqpkVFYajaLnU/oESh5SAUz9bD6hHZFyeseSBl89AHXkgcgezdixQ8vzbzXVogXSjxkbaNQl+ErCpHFVpJbEiJO9CifHzRpPBqApJAdVnKgcqNfd84CIw03BFOhVLd9IMzf/BKEjJyCIewGx/WJenSQxjCZYtLCXQeRecAYboY5MP64rPNO12UYOn6a+V9pdMZgxW2w05Ag+wC+8auF+Lbrln7PlLrtIqvGXbpFxtqTI8yvBcFM2OfXy X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 12 Mar 2026 04:47:28.5320 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 3e111d46-194b-486b-1253-08de7ff276e4 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: MWH0EPF000C6184.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR12MB4350 Content-Type: text/plain; charset="utf-8" Use the "sd_llc" passed to select_idle_cpu() to obtain the "sd_llc_shared" instead of dereferencing the per-CPU variable. Since "sd->shared" is always reclaimed at the same time as "sd" via call_rcu() and update_top_cache_domain() always ensures a valid "sd->shared" assignment when "sd_llc" is present, "sd_llc->shared" can always be dereferenced without needing an additional check. While at it move the cpumask_and() operation after the SIS_UTIL bailout check to avoid unnecessarily computing the cpumask. Reviewed-by: Chen Yu Reviewed-by: Shrikanth Hegde Signed-off-by: K Prateek Nayak --- Changelog v3..v4: o Collected tags from Chenyu, and Shrikanth. (Thanks a ton!) o Added a brief comment in select_idle_cpu() on why directly dereferencing "sd->shared" is safe as long as an RCU-protected reference to "sd" exists. (Shrikanth). --- kernel/sched/fair.c | 27 ++++++++++++++++----------- 1 file changed, 16 insertions(+), 11 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d7e4de909a63..8dbf63d460b8 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7876,21 +7876,26 @@ static int select_idle_cpu(struct task_struct *p, s= truct sched_domain *sd, bool { struct cpumask *cpus =3D this_cpu_cpumask_var_ptr(select_rq_mask); int i, cpu, idle_cpu =3D -1, nr =3D INT_MAX; - struct sched_domain_shared *sd_share; - - cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr); =20 if (sched_feat(SIS_UTIL)) { - sd_share =3D rcu_dereference_all(per_cpu(sd_llc_shared, target)); - if (sd_share) { - /* because !--nr is the condition to stop scan */ - nr =3D READ_ONCE(sd_share->nr_idle_scan) + 1; - /* overloaded LLC is unlikely to have idle cpu/core */ - if (nr =3D=3D 1) - return -1; - } + /* + * Increment because !--nr is the condition to stop scan. + * + * Since "sd" is "sd_llc" for target CPU dereferenced in the + * caller, it is safe to directly dereference "sd->shared". + * Topology bits always ensure it assigned for "sd_llc" abd it + * cannot disappear as long as we have a RCU protected + * reference to one the associated "sd" here. + */ + nr =3D READ_ONCE(sd->shared->nr_idle_scan) + 1; + /* overloaded LLC is unlikely to have idle cpu/core */ + if (nr =3D=3D 1) + return -1; } =20 + if (!cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr)) + return -1; + if (static_branch_unlikely(&sched_cluster_active)) { struct sched_group *sg =3D sd->groups; =20 --=20 2.34.1