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sched_ext: Fix starvation of scx_enable() under fair-class saturation

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During scx_enable(), the READY -> ENABLED task switching loop changes the
calling thread's sched_class from fair to ext. Since fair has higher
priority than ext, saturating fair-class workloads can indefinitely starve
the enable thread, hanging the system. This was introduced when the enable
path switched from preempt_disable() to scx_bypass() which doesn't protect
against fair-class starvation. Note that the original preempt_disable()
protection wasn't complete either - in partial switch modes, the calling
thread could still be starved after preempt_enable() as it may have been
switched to ext class.

Fix it by offloading the enable body to a dedicated system-wide RT
(SCHED_FIFO) kthread which cannot be starved by either fair or ext class
tasks. scx_enable() lazily creates the kthread on first use and passes the
ops pointer through a struct scx_enable_cmd containing the kthread_work,
then synchronously waits for completion.

The workfn runs on a different kthread from sch->helper (which runs
disable_work), so it can safely flush disable_work on the error path
without deadlock.

Fixes: 8c2090c504 ("sched_ext: Initialize in bypass mode")
Cc: stable@vger.kernel.org # v6.12+
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo 2026-03-03 01:01:15 -10:00
parent 1336b579f6
commit b06ccbabe2
1 changed files with 56 additions and 10 deletions
Download patch file Download diff file Expand all files Collapse all files

66
kernel/sched/ext.c
View file

@ -4975,20 +4975,30 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
return 0;
}
static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
/*
* scx_enable() is offloaded to a dedicated system-wide RT kthread to avoid
* starvation. During the READY -> ENABLED task switching loop, the calling
* thread's sched_class gets switched from fair to ext. As fair has higher
* priority than ext, the calling thread can be indefinitely starved under
* fair-class saturation, leading to a system hang.
*/
struct scx_enable_cmd {
struct kthread_work work;
struct sched_ext_ops *ops;
int ret;
};
static void scx_enable_workfn(struct kthread_work *work)
{
struct scx_enable_cmd *cmd =
container_of(work, struct scx_enable_cmd, work);
struct sched_ext_ops *ops = cmd->ops;
struct scx_sched *sch;
struct scx_task_iter sti;
struct task_struct *p;
unsigned long timeout;
int i, cpu, ret;
if (!cpumask_equal(housekeeping_cpumask(HK_TYPE_DOMAIN),
cpu_possible_mask)) {
pr_err("sched_ext: Not compatible with \"isolcpus=\" domain isolation\n");
return -EINVAL;
}
mutex_lock(&scx_enable_mutex);
if (scx_enable_state() != SCX_DISABLED) {
@ -5205,13 +5215,15 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
atomic_long_inc(&scx_enable_seq);
return 0;
cmd->ret = 0;
return;
err_free_ksyncs:
free_kick_syncs();
err_unlock:
mutex_unlock(&scx_enable_mutex);
return ret;
cmd->ret = ret;
return;
err_disable_unlock_all:
scx_cgroup_unlock();
@ -5230,7 +5242,41 @@ err_disable:
*/
scx_error(sch, "scx_enable() failed (%d)", ret);
kthread_flush_work(&sch->disable_work);
return 0;
cmd->ret = 0;
}
static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
{
static struct kthread_worker *helper;
static DEFINE_MUTEX(helper_mutex);
struct scx_enable_cmd cmd;
if (!cpumask_equal(housekeeping_cpumask(HK_TYPE_DOMAIN),
cpu_possible_mask)) {
pr_err("sched_ext: Not compatible with \"isolcpus=\" domain isolation\n");
return -EINVAL;
}
if (!READ_ONCE(helper)) {
mutex_lock(&helper_mutex);
if (!helper) {
helper = kthread_run_worker(0, "scx_enable_helper");
if (IS_ERR_OR_NULL(helper)) {
helper = NULL;
mutex_unlock(&helper_mutex);
return -ENOMEM;
}
sched_set_fifo(helper->task);
}
mutex_unlock(&helper_mutex);
}
kthread_init_work(&cmd.work, scx_enable_workfn);
cmd.ops = ops;
kthread_queue_work(READ_ONCE(helper), &cmd.work);
kthread_flush_work(&cmd.work);
return cmd.ret;
}