From: Tejun Heo <tj@kernel.org> mainline inclusion from mainline-v6.12-rc1 commit 8c2090c504e998c8f34ec870bae71dafcc96a6e0 category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/IDC9YK Reference: https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commi... -------------------------------- scx_ops_enable() used preempt_disable() around the task iteration loop to switch tasks into SCX to guarantee forward progress of the task which is running scx_ops_enable(). However, in the gap between setting __scx_ops_enabled and preeempt_disable(), an external entity can put tasks including the enabling one into SCX prematurely, which can lead to malfunctions including stalls. The bypass mode can wrap the entire enabling operation and guarantee forward progress no matter what the BPF scheduler does. Use the bypass mode instead to guarantee forward progress while enabling. While at it, release and regrab scx_tasks_lock between the two task iteration locks in scx_ops_enable() for clarity as there is no reason to keep holding the lock between them. Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Zicheng Qu <quzicheng@huawei.com> --- kernel/sched/ext.c | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index c97fed31a576..c03d398f4248 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -5083,6 +5083,14 @@ static int scx_ops_enable(struct sched_ext_ops *ops) queue_delayed_work(system_unbound_wq, &scx_watchdog_work, scx_watchdog_timeout / 2); + /* + * Once __scx_ops_enabled is set, %current can be switched to SCX + * anytime. This can lead to stalls as some BPF schedulers (e.g. + * userspace scheduling) may not function correctly before all tasks are + * switched. Init in bypass mode to guarantee forward progress. + */ + scx_ops_bypass(true); + /* * Lock out forks, cgroup on/offlining and moves before opening the * floodgate so that they don't wander into the operations prematurely. @@ -5142,7 +5150,6 @@ static int scx_ops_enable(struct sched_ext_ops *ops) * disabled. */ spin_lock_irq(&scx_tasks_lock); - scx_task_iter_init(&sti); while ((p = scx_task_iter_next_locked(&sti))) { /* @@ -5171,22 +5178,19 @@ static int scx_ops_enable(struct sched_ext_ops *ops) spin_lock_irq(&scx_tasks_lock); } scx_task_iter_exit(&sti); + spin_unlock_irq(&scx_tasks_lock); /* - * All tasks are prepped but are still ops-disabled. Ensure that - * %current can't be scheduled out and switch everyone. - * preempt_disable() is necessary because we can't guarantee that - * %current won't be starved if scheduled out while switching. + * All tasks are prepped but the tasks are not enabled. Switch everyone. */ - preempt_disable(); + WRITE_ONCE(scx_switching_all, !(ops->flags & SCX_OPS_SWITCH_PARTIAL)); /* * We're fully committed and can't fail. The PREPPED -> ENABLED * transitions here are synchronized against sched_ext_free() through * scx_tasks_lock. */ - WRITE_ONCE(scx_switching_all, !(ops->flags & SCX_OPS_SWITCH_PARTIAL)); - + spin_lock_irq(&scx_tasks_lock); scx_task_iter_init(&sti); while ((p = scx_task_iter_next_locked(&sti))) { const struct sched_class *old_class = p->sched_class; @@ -5203,12 +5207,12 @@ static int scx_ops_enable(struct sched_ext_ops *ops) check_class_changed(task_rq(p), p, old_class, p->prio); } scx_task_iter_exit(&sti); - spin_unlock_irq(&scx_tasks_lock); - preempt_enable(); + scx_cgroup_unlock(); cpus_read_unlock(); percpu_up_write(&scx_fork_rwsem); + scx_ops_bypass(false); /* * Returning an error code here would lose the recorded error @@ -5249,6 +5253,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops) err_disable_unlock_all: scx_cgroup_unlock(); percpu_up_write(&scx_fork_rwsem); + scx_ops_bypass(false); err_disable_unlock_cpus: cpus_read_unlock(); err_disable: -- 2.34.1