On Tue Dec 9, 2025 at 4:04 PM EST, Tejun Heo wrote:

scx_enable() calls scx_bypass(true) to initialize in bypass mode and then scx_bypass(false) on success to exit. If scx_enable() fails during task initialization - e.g. scx_cgroup_init() or scx_init_task() returns an error - it jumps to err_disable while bypass is still active. scx_disable_workfn() then calls scx_bypass(true/false) for its own bypass, leaving the bypass depth at 1 instead of 0. This causes the system to remain permanently in bypass mode after a failed scx_enable().

Failures after task initialization is complete - e.g. scx_tryset_enable_state() at the end - already call scx_bypass(false) before reaching the error path and are not affected. This only affects a subset of failure modes.

Fix it by tracking whether scx_enable() called scx_bypass(true) in a bool and having scx_disable_workfn() call an extra scx_bypass(false) to clear it. This is a temporary measure as the bypass depth will be moved into the sched instance, which will make this tracking unnecessary.

Fixes: 8c2090c504e9 ("sched_ext: Initialize in bypass mode") Cc: stable@vger.kernel.org # v6.12+ Reported-by: Chris Mason clm@meta.com Signed-off-by: Tejun Heo tj@kernel.org

Reviewed-by: Emil Tsalapatis emil@etsalapatis.com

---

kernel/sched/ext.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)

--- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -41,6 +41,13 @@ static bool scx_init_task_enabled; static bool scx_switching_all; DEFINE_STATIC_KEY_FALSE(__scx_switched_all);

+/*

• • Tracks whether scx_enable() called scx_bypass(true). Used to balance bypass
• • depth on enable failure. Will be removed when bypass depth is moved into the
• • sched instance.
• */

+static bool scx_bypassed_for_enable;

static atomic_long_t scx_nr_rejected = ATOMIC_LONG_INIT(0); static atomic_long_t scx_hotplug_seq = ATOMIC_LONG_INIT(0);

@@ -4318,6 +4325,11 @@ static void scx_disable_workfn(struct kt scx_dsp_max_batch = 0; free_kick_syncs();

• if (scx_bypassed_for_enable) {

• scx_bypassed_for_enable = false;
  

• scx_bypass(false);
  

• }

• mutex_unlock(&scx_enable_mutex);

  WARN_ON_ONCE(scx_set_enable_state(SCX_DISABLED) != SCX_DISABLING);

@@ -4970,6 +4982,7 @@ static int scx_enable(struct sched_ext_o * Init in bypass mode to guarantee forward progress. */ scx_bypass(true);

• scx_bypassed_for_enable = true;

  for (i = SCX_OPI_NORMAL_BEGIN; i < SCX_OPI_NORMAL_END; i++) if (((void (**)(void))ops)[i])

@@ -5067,6 +5080,7 @@ static int scx_enable(struct sched_ext_o scx_task_iter_stop(&sti); percpu_up_write(&scx_fork_rwsem);

• scx_bypassed_for_enable = false; scx_bypass(false);

  if (!scx_tryset_enable_state(SCX_ENABLED, SCX_ENABLING)) {

-- tejun