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 Cc: stable@vger.kernel.org Signed-off-by: Andrea Righi arighi@nvidia.com --- kernel/sched/ext.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index c801dd20c63d9..7eae1c64f7348 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.