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Tejun Heo
authored
commit f3f08c3acfb8860e07a22814a344e83c99ad7398 upstream. While fixing migration disabled task handling, 32966821574c ("sched_ext: Fix migration disabled handling in targeted dispatches") assumed that a migration disabled task's ->cpus_ptr would only have the pinned CPU. While this is eventually true for migration disabled tasks that are switched out, ->cpus_ptr update is performed by migrate_disable_switch() which is called right before context_switch() in __scheduler(). However, the task is enqueued earlier during pick_next_task() via put_prev_task_scx(), so there is a race window where another CPU can see the task on a DSQ. If the CPU tries to dispatch the migration disabled task while in that window, task_allowed_on_cpu() will succeed and task_can_run_on_remote_rq() will subsequently trigger SCHED_WARN(is_migration_disabled()). WARNING: CPU: 8 PID: 1837 at kernel/sched/ext.c:2466 task_can_run_on_remote_rq+0x12e/0x140 Sched_ext: layered (enabled+all), task: runnable_at=-10ms RIP: 0010:task_can_run_on_remote_rq+0x12e/0x140 ... <TASK> consume_dispatch_q+0xab/0x220 scx_bpf_dsq_move_to_local+0x58/0xd0 bpf_prog_84dd17b0654b6cf0_layered_dispatch+0x290/0x1cfa bpf__sched_ext_ops_dispatch+0x4b/0xab balance_one+0x1fe/0x3b0 balance_scx+0x61/0x1d0 prev_balance+0x46/0xc0 __pick_next_task+0x73/0x1c0 __schedule+0x206/0x1730 schedule+0x3a/0x160 __do_sys_sched_yield+0xe/0x20 do_syscall_64+0xbb/0x1e0 entry_SYSCALL_64_after_hwframe+0x77/0x7f Fix it by converting the SCHED_WARN() back to a regular failure path. Also, perform the migration disabled test before task_allowed_on_cpu() test so that BPF schedulers which fail to handle migration disabled tasks can be noticed easily. While at it, adjust scx_ops_error() message for !task_allowed_on_cpu() case for brevity and consistency. Signed-off-by:Tejun Heo <tj@kernel.org> Fixes: 32966821574c ("sched_ext: Fix migration disabled handling in targeted dispatches") Acked-by:
Andrea Righi <arighi@nvidia.com> Reported-by:
Jake Hillion <jakehillion@meta.com> Signed-off-by:
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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