[PATCH 1/2] sched_ext: Initialize idle masks as busy

[Andrea Righi <arighi@nvidia.com>]

The built-in idle masks are reset with all online CPUs marked idle
before sched_ext is enabled. Busy CPUs can therefore be incorrectly
advertised as idle until their next idle transition.

Initialize the masks empty so that the initial state is conservative.
When bypass is lifted, every CPU is rescheduled and idle-to-idle
re-picks populate the masks with CPUs that are actually idle. Later
idle transitions keep the masks up to date.

Cc: Kuba Piecuch <jpiecuch@google.com>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext/idle.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/ext/idle.c b/kernel/sched/ext/idle.c
index 3e9d6a44bf431..33bd51ab3cede 100644
--- a/kernel/sched/ext/idle.c
+++ b/kernel/sched/ext/idle.c
@@ -838,20 +838,20 @@ static void reset_idle_masks(struct sched_ext_ops *ops)
 	int node;
 
 	/*
-	 * Consider all online cpus idle. Should converge to the actual state
-	 * quickly.
+	 * Start with all CPUs marked busy. The idle masks are populated when
+	 * bypass is lifted and each idle CPU is forced through an idle re-pick.
+	 * This may temporarily omit idle CPUs but never advertises a busy CPU as
+	 * idle.
 	 */
 	if (!(ops->flags & SCX_OPS_BUILTIN_IDLE_PER_NODE)) {
-		cpumask_copy(idle_cpumask(NUMA_NO_NODE)->cpu, cpu_online_mask);
-		cpumask_copy(idle_cpumask(NUMA_NO_NODE)->smt, cpu_online_mask);
+		cpumask_clear(idle_cpumask(NUMA_NO_NODE)->cpu);
+		cpumask_clear(idle_cpumask(NUMA_NO_NODE)->smt);
 		return;
 	}
 
 	for_each_node(node) {
-		const struct cpumask *node_mask = cpumask_of_node(node);
-
-		cpumask_and(idle_cpumask(node)->cpu, cpu_online_mask, node_mask);
-		cpumask_and(idle_cpumask(node)->smt, cpu_online_mask, node_mask);
+		cpumask_clear(idle_cpumask(node)->cpu);
+		cpumask_clear(idle_cpumask(node)->smt);
 	}
 }
 
-- 
2.55.0