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sched_ext: Mark waker CPU busy when selected in WAKE_SYNC case

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SCX's built-in idle CPU tracking is imperfect and can be out-of-sync
with the actual idle state of CPUs, especially immediately after
enabling SCX due to scx_idle_enable() marking all online CPUs idle.

scx_select_cpu_dfl() skips marking the selected CPU as busy if
the selected CPU is the waker CPU in the SCX_WAKE_SYNC case.
If the waker CPU was marked idle by SCX, it will still be marked idle
after CPU selection and potentially even after switching to the wakee.

In the allowed_cpus selftest, this can manifest as the test failing with
the following message in dmesg:

  allowed_cpus.bpf.c:21: CPU 0 should be marked as busy

This patch explicitly marks the waker CPU as busy. With this patch,
the test failure no longer reproduces. There are still some pretty
unlikely races that could make the test fail (e.g. pick_task_idle()
marking the selected CPU idle between selection and validation), but
these can't be fixed easily.

Signed-off-by: Kuba Piecuch <jpiecuch@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Kuba Piecuch
2026-07-22 14:33:07 +00:00
committed by Tejun Heo
parent 5cdc928598
commit 9591fcc95d
1 changed files with 3 additions and 1 deletions
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4
kernel/sched/ext/idle.c
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@@ -554,8 +554,10 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
cpu_rq(cpu)->scx.local_dsq.nr == 0 &&
(!(flags & SCX_PICK_IDLE_IN_NODE) || (waker_node == node)) &&
!cpumask_empty(idle_cpumask(waker_node)->cpu)) {
if (cpumask_test_cpu(cpu, allowed))
if (cpumask_test_cpu(cpu, allowed)) {
scx_idle_test_and_clear_cpu(cpu);
goto out_unlock;
}
}
}