sched_ext: initialize built-in idle state before ops.init()

commit f0c6eab5e45c529f449fbc595873719e00de6d79 upstream.

A BPF scheduler may want to use the built-in idle cpumasks in ops.init()
before the scheduler is fully initialized, either directly or through a
BPF timer for example.

However, this would result in an error, since the idle state has not
been properly initialized yet.

This can be easily verified by modifying scx_simple to call
scx_bpf_get_idle_cpumask() in ops.init():

$ sudo scx_simple

DEBUG DUMP
===========================================================================

scx_simple[121] triggered exit kind 1024:
  runtime error (built-in idle tracking is disabled)
...

Fix this by properly initializing the idle state before ops.init() is
called. With this change applied:

$ sudo scx_simple
local=2 global=0
local=19 global=11
local=23 global=11
...

Fixes: d73249f88743d ("sched_ext: idle: Make idle static keys private")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[ Backport to 6.12:
  - Original commit doesn't apply cleanly to 6.12 since d73249f88743d is
    not present.
  - This backport applies the same logical fix to prevent BPF scheduler
    failures while accessing idle cpumasks from ops.init(). ]
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 304bb894857b..563a7dc2ece6 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5220,6 +5220,13 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	for_each_possible_cpu(cpu)
 		cpu_rq(cpu)->scx.cpuperf_target = SCX_CPUPERF_ONE;
 
+	if (!ops->update_idle || (ops->flags & SCX_OPS_KEEP_BUILTIN_IDLE)) {
+		reset_idle_masks();
+		static_branch_enable(&scx_builtin_idle_enabled);
+	} else {
+		static_branch_disable(&scx_builtin_idle_enabled);
+	}
+
 	/*
 	 * Keep CPUs stable during enable so that the BPF scheduler can track
 	 * online CPUs by watching ->on/offline_cpu() after ->init().
@@ -5287,13 +5294,6 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	if (scx_ops.cpu_acquire || scx_ops.cpu_release)
 		static_branch_enable(&scx_ops_cpu_preempt);
 
-	if (!ops->update_idle || (ops->flags & SCX_OPS_KEEP_BUILTIN_IDLE)) {
-		reset_idle_masks();
-		static_branch_enable(&scx_builtin_idle_enabled);
-	} else {
-		static_branch_disable(&scx_builtin_idle_enabled);
-	}
-
 	/*
 	 * Lock out forks, cgroup on/offlining and moves before opening the
 	 * floodgate so that they don't wander into the operations prematurely.