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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?y2BMw/Yz4e+2XkO7CO9L/tI4+4RM0SCWEDn12MzUAIJuItlHiwsJSlfTI9ta?= =?us-ascii?Q?EV6I0hVQfuAq1TGiSj75v9FC9WSILa+Qg8FmDxpu+C6nsyjyDcOMdAeMf96R?= =?us-ascii?Q?tVRbEFyZzntYV0/L8syCWJ1p0mfhUyOqvGEx+vvsONMVbLZ8n6XADYkKQrLm?= =?us-ascii?Q?ITuYWxTR3RM4zs0aG95kkCCMCAOWpVttrJimoku4Ni72ggzhcV3RKmcFcl05?= =?us-ascii?Q?hzPxF17JCq0CgYaZmLHmi31Or9vGjYW2fLO78fNllkjj7L1iUBCGzDcY5o6l?= =?us-ascii?Q?zqdUY/fFocyFL99iUjW0q985+oKMOvtSkuFxajywSp/wXITUtMaYmztz4CRj?= =?us-ascii?Q?6cZDoylglvEHnOk/gDy4ZgERd9T2MypoMVGvylvffCrAe9/VIH9J9mVXYAuz?= =?us-ascii?Q?/u50mW8n/R+7ChgDpx938RYjzMbRGpeuo0Sl23cmPZZMx00ERCUThctTKSOs?= =?us-ascii?Q?/nH83xnV4T11Y/Vd3Jp8I4Ol0D32fXZbuqQ4qpF/N2M/z0Q8TbgV/I1NrnJJ?= =?us-ascii?Q?9pgIAR+v8LAV2MUDBQEGmHy4b7+lmSF/p8a0BXIKtOtPVneSJdAzvhiwg77b?= =?us-ascii?Q?7BFj29gXCFOjpcVV+CLNBE9Rsqi0EAhv/9+nwraHBvBYhSiIYE2g3ihFxzQG?= =?us-ascii?Q?ayfBG7iE4ISxtuKTWm+v+rCE+wih7NYtwvk62fA9uSZb5gjaUGDnSMwMq3AJ?= =?us-ascii?Q?i+Q2YJFpEGgVO8Yd9uqddtK0rHU4QQNRQSAsQmBKAlNgKo0u04a2MxEZr1Ye?= =?us-ascii?Q?r76Sdk09b98ELLtHazCL2IfjyHKDS2JuO5mldnTQ0C3CHV3S0W8auMDmbKzm?= =?us-ascii?Q?cqvfjee1f8S6sF1unRPK6ttF8Fo/PdqZdIMcfZZnLbB7mxEqISqixNECrNQV?= =?us-ascii?Q?InnueLnUJhx07n8Z0juAuAn9awcjDKFE6/VTTthMnDKnl1S2zxcUliBFrZaq?= =?us-ascii?Q?kRePwq5cMC1O+FNq5It1QAJx57zTrRxcm+mJkzqH0txwNk5LsS64MD3Y4BIt?= =?us-ascii?Q?9h9AWZUxotlbMoTD2nvMSUO0xbBhNYbTONS4aR8w4Os4m9d+UrUnjv0LiMWb?= =?us-ascii?Q?pp2iewX8TKLn2XEmBS5X+KaPkZe+c9RUO2N1nqO2XxT5unrox9cYAaZX4PKL?= =?us-ascii?Q?8UeugdAdo3U6LnKosyeLkfSkguFob/jW+0SlDDJfbFEGCDNOFib+PPsstVIp?= =?us-ascii?Q?W5XXDjOgzeG0FasFEEGDXR5zYnRKMp0CrPU0uxTagwluww5tKIOEKC/gU6kM?= =?us-ascii?Q?Flx/Mk8n5L6csxWr+5dCmtVgnPVSDt3F5B03TZNmG8lIutzyZscwmr3VBZtO?= =?us-ascii?Q?x2Vc+M5akfvxFBC4ppX0T4EYttRnizAxBi+2WuIdI/rWVdbHEAb6Fu6nuXfB?= =?us-ascii?Q?JJCohTqSnKjW4yJKVIvnWzJiYGAk1WoIR29FF1GzDQ8ppuugSXjWizuvQQOW?= =?us-ascii?Q?1CKqZvN7+93uJ8i+DqY//TRAVwTguFeYbsKlXojZrQ+bdMV/JdDHYhc7PP9o?= =?us-ascii?Q?hXJ05SZ9Qb+usLNcJ7WvOnJfwckvZPYciyYeqSVjHdzhW7arEJKucUW6U7Db?= =?us-ascii?Q?CG0zP6EgZRPQgQK/BonrPCGqE/Dl6jrfw6S53nlkq98K+LyLbB0WJK86kPwi?= =?us-ascii?Q?u4aqHl2updYq7orDF1b/dhfSd8adfcq3p0A50h/BcMLi26krv45s0x9dUU5+?= =?us-ascii?Q?E9x7VWW5py5ISqQrxaslzdOD+CbZRMw6tuWYr7LuMpUyeVZYNwBF6xhg/tn1?= =?us-ascii?Q?dRF1RKZk8A=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 55c49b18-579d-49dc-20f8-08dea80df1e9 X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 May 2026 05:44:58.3590 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: iVv/xao1Ue/zDo0J3QmkK4wBLLCTNLLhXTZHR0hv3diG3ajQCWW4nv1bjM9zWa5K/5C+THd4HeZKg+oGx6Fa2Q== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DSWPR12MB999154 On Thu, Apr 30, 2026 at 10:27:47AM +0100, David Carlier wrote: > scx_select_cpu_dfl() narrows @allowed to @cpus_allowed & @p->cpus_ptr > when the BPF caller supplies a @cpus_allowed that differs from > @p->cpus_ptr and @p doesn't have full affinity. However, > @is_prev_allowed was computed against the original (wider) > @cpus_allowed, so the prev_cpu fast paths could pick a @prev_cpu that > is in @cpus_allowed but not in @p->cpus_ptr, violating the intended > invariant that the returned CPU is always usable by @p. The kernel > masks this via the SCX_EV_SELECT_CPU_FALLBACK fallback, but the > behavior contradicts the documented contract. > > Move the @is_prev_allowed evaluation past the narrowing block so it > tests against the final @allowed mask. > > Fixes: ee9a4e92799d ("sched_ext: idle: Properly handle invalid prev_cpu during idle selection") > Assisted-by: Claude > Signed-off-by: David Carlier Makes sense, good catch. Reviewed-by: Andrea Righi Thanks, -Andrea > --- > kernel/sched/ext_idle.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c > index 41785f65bbb2..bbb845f36a0a 100644 > --- a/kernel/sched/ext_idle.c > +++ b/kernel/sched/ext_idle.c > @@ -464,12 +464,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, > > preempt_disable(); > > - /* > - * Check whether @prev_cpu is still within the allowed set. If not, > - * we can still try selecting a nearby CPU. > - */ > - is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed); > - > /* > * Determine the subset of CPUs usable by @p within @cpus_allowed. > */ > @@ -486,6 +480,12 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, > } > } > > + /* > + * Check whether @prev_cpu is still within the allowed set. If not, > + * we can still try selecting a nearby CPU. > + */ > + is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed); > + > /* > * This is necessary to protect llc_cpus. > */ > -- > 2.53.0 >