From: Tejun Heo Date: Mon, 3 Nov 2025 20:25:13 +0000 (-1000) Subject: sched_ext: Fix cgroup exit ordering by moving sched_ext_free() to finish_task_switch() X-Git-Tag: v6.19-rc1~177^2~21 X-Git-Url: https://gentwo.org/gitweb/?a=commitdiff_plain;h=7900aa699c34401cf5d0c701d9ef72880ddc1a83;p=linux%2F.git sched_ext: Fix cgroup exit ordering by moving sched_ext_free() to finish_task_switch() sched_ext_free() was called from __put_task_struct() when the last reference to the task is dropped, which could be long after the task has finished running. This causes cgroup-related problems: - ops.init_task() can be called on a cgroup which didn't get ops.cgroup_init()'d during scheduler load, because the cgroup might be destroyed/unlinked while the zombie or dead task is still lingering on the scx_tasks list. - ops.cgroup_exit() could be called before ops.exit_task() is called on all member tasks, leading to incorrect exit ordering. Fix by moving it to finish_task_switch() to be called right after the final context switch away from the dying task, matching when sched_class->task_dead() is called. Rename it to sched_ext_dead() to match the new calling context. By calling sched_ext_dead() before cgroup_task_dead(), we ensure that: - Tasks visible on scx_tasks list have valid cgroups during scheduler load, as cgroup_mutex prevents cgroup destruction while the task is still linked. - All member tasks have ops.exit_task() called and are removed from scx_tasks before the cgroup can be destroyed and trigger ops.cgroup_exit(). This fix is made possible by the cgroup_task_dead() split in the previous patch. This also makes more sense resource-wise as there's no point in keeping scheduler side resources around for dead tasks. Reported-by: Dan Schatzberg Cc: Peter Zijlstra Reviewed-by: Andrea Righi Signed-off-by: Tejun Heo --- diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h index 4713f374acc0..eb776b094d36 100644 --- a/include/linux/sched/ext.h +++ b/include/linux/sched/ext.h @@ -208,14 +208,14 @@ struct sched_ext_entity { struct list_head tasks_node; }; -void sched_ext_free(struct task_struct *p); +void sched_ext_dead(struct task_struct *p); void print_scx_info(const char *log_lvl, struct task_struct *p); void scx_softlockup(u32 dur_s); bool scx_rcu_cpu_stall(void); #else /* !CONFIG_SCHED_CLASS_EXT */ -static inline void sched_ext_free(struct task_struct *p) {} +static inline void sched_ext_dead(struct task_struct *p) {} static inline void print_scx_info(const char *log_lvl, struct task_struct *p) {} static inline void scx_softlockup(u32 dur_s) {} static inline bool scx_rcu_cpu_stall(void) { return false; } diff --git a/kernel/fork.c b/kernel/fork.c index 960c39c9c264..5ae37909a813 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -736,7 +736,6 @@ void __put_task_struct(struct task_struct *tsk) WARN_ON(tsk == current); unwind_task_free(tsk); - sched_ext_free(tsk); io_uring_free(tsk); cgroup_task_free(tsk); task_numa_free(tsk, true); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 0324457622d7..3f4653106216 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5151,6 +5151,12 @@ static struct rq *finish_task_switch(struct task_struct *prev) if (prev->sched_class->task_dead) prev->sched_class->task_dead(prev); + /* + * sched_ext_dead() must come before cgroup_task_dead() to + * prevent cgroups from being removed while its member tasks are + * visible to SCX schedulers. + */ + sched_ext_dead(prev); cgroup_task_dead(prev); /* Task is done with its stack. */ diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index d1ef5bda95ae..2811e4f42a37 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -2966,7 +2966,7 @@ void scx_cancel_fork(struct task_struct *p) percpu_up_read(&scx_fork_rwsem); } -void sched_ext_free(struct task_struct *p) +void sched_ext_dead(struct task_struct *p) { unsigned long flags;