[PATCH 6/6] sched_ext: Clear the built-in idle masks on CPU hotplug transitions

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

The built-in idle masks are only maintained by __scx_update_idle()
(task driven idle transitions) and reset_idle_masks() (ops load);
nothing clears them when a CPU is hotplugged. If a CPU goes offline
while idle, its bit stays set for the whole offline window:
scx_bpf_pick_idle_cpu() and the other idle kfuncs keep reporting the
offline CPU as an idle candidate, and the built-in select_cpu path may
claim it once before the guard layers bounce the wakeup, costing a
failed pick and a bumped SCX_EV_SELECT_CPU_FALLBACK each time. This is
not the documented racy and self-correcting regime of the idle SMT
handling: the stale bit is a deterministic leftover, and read-only
kfunc consumers never trigger the claim-time self-correction.

handle_hotplug() now drops the CPU from the built-in idle masks on
both transitions like it already does for the cid-form online_cmask.
scx_idle_test_and_clear_cpu() is also gated on cpu_online(): the clear
alone is not sufficient, because a dying CPU keeps re-picking its idle
task until it halts and each re-pick re-sets its bit, which can happen
after the hotplug paths have already run. The claim path is the one
choke point every idle consumer goes through, so an offline CPU is
never handed out as an idle candidate regardless of any stale bits.

Verified in qemu (4 cpus) with an idle-probe scheduler probing
scx_bpf_pick_idle_cpu() every dispatch: 15/15 offline/online cycles
probe zero stale entries with the change. Stock is not A/B-comparable,
as every hotplug transition restarts the scheduler and reinitializes
the masks; the fix rests on the path above.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 kernel/sched/ext/ext.c  |  7 +++++++
 kernel/sched/ext/idle.c | 21 +++++++++++++++++++++
 kernel/sched/ext/idle.h |  1 +
 3 files changed, 29 insertions(+)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 792cf0c1d582..a9612d768986 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4356,6 +4356,13 @@ static void handle_hotplug(struct rq *rq, bool online)
 	if (scx_enabled())
 		scx_idle_update_selcpu_topology(&sch->ops);
 
+	/*
+	 * Keep the built-in idle masks in sync with the hotplug state: an
+	 * offline CPU must stop being advertised as idle and a CPU coming
+	 * back must not inherit a stale idle bit.
+	 */
+	scx_idle_clear_cpu(cpu);
+
 	/*
 	 * The tables can't be retired while this function is running as the
 	 * retirement is inside cpus_read_lock. However, scx_cpu_arg() is
diff --git a/kernel/sched/ext/idle.c b/kernel/sched/ext/idle.c
index 922fbc3065cc..ff0c5402886e 100644
--- a/kernel/sched/ext/idle.c
+++ b/kernel/sched/ext/idle.c
@@ -107,6 +107,18 @@ static bool scx_idle_test_and_clear_cpu(int cpu)
 			__cpumask_clear_cpu(cpu, idle_smts);
 	}
 
+	/*
+	 * An offline CPU is never an idle candidate, but its bit can
+	 * linger past @cpu going offline: the dying CPU keeps re-picking
+	 * its idle task until it halts, which can happen after the
+	 * hotplug paths have already run. Drop the stale bit and refuse
+	 * the claim.
+	 */
+	if (unlikely(!cpu_online(cpu))) {
+		cpumask_clear_cpu(cpu, idle_cpus);
+		return false;
+	}
+
 	return cpumask_test_and_clear_cpu(cpu, idle_cpus);
 }
 
@@ -743,6 +755,15 @@ static void update_builtin_idle(int cpu, bool idle)
 	}
 }
 
+/*
+ * Drop @cpu from the built-in idle masks; used on hotplug transitions
+ * so an offline CPU stops being advertised as idle.
+ */
+void scx_idle_clear_cpu(s32 cpu)
+{
+	update_builtin_idle(cpu, false);
+}
+
 /*
  * Notify schedulers of an idle transition on @cpu's cid, delivering to every
  * sched that holds %SCX_CAP_BASE on the cid (the root holds every cap). A real
diff --git a/kernel/sched/ext/idle.h b/kernel/sched/ext/idle.h
index 87a0e58f1eb7..89180abbfd21 100644
--- a/kernel/sched/ext/idle.h
+++ b/kernel/sched/ext/idle.h
@@ -21,6 +21,7 @@ extern struct btf_id_set8 scx_kfunc_ids_select_cpu;
 
 void scx_idle_update_selcpu_topology(struct sched_ext_ops *ops);
 void scx_idle_init_masks(void);
+void scx_idle_clear_cpu(s32 cpu);
 
 s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 		       const struct cpumask *cpus_allowed, u64 flags);
-- 
2.53.0

base-commit: 5b740a72b55c4267ba4d8010f17dde26b26f52d3