sched_ext: Fix incorrect autogroup migration detection

scx_move_task() is called from sched_move_task() and tells the BPF scheduler
that cgroup migration is being committed. sched_move_task() is used by both
cgroup and autogroup migrations and scx_move_task() tried to filter out
autogroup migrations by testing the destination cgroup and PF_EXITING but
this is not enough. In fact, without explicitly tagging the thread which is
doing the cgroup migration, there is no good way to tell apart
scx_move_task() invocations for racing migration to the root cgroup and an
autogroup migration.

This led to scx_move_task() incorrectly ignoring a migration from non-root
cgroup to an autogroup of the root cgroup triggering the following warning:

  WARNING: CPU: 7 PID: 1 at kernel/sched/ext.c:3725 scx_cgroup_can_attach+0x196/0x340
  ...
  Call Trace:
  <TASK>
    cgroup_migrate_execute+0x5b1/0x700
    cgroup_attach_task+0x296/0x400
    __cgroup_procs_write+0x128/0x140
    cgroup_procs_write+0x17/0x30
    kernfs_fop_write_iter+0x141/0x1f0
    vfs_write+0x31d/0x4a0
    __x64_sys_write+0x72/0xf0
    do_syscall_64+0x82/0x160
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fix it by adding an argument to sched_move_task() that indicates whether the
moving is for a cgroup or autogroup migration. After the change,
scx_move_task() is called only for cgroup migrations and renamed to
scx_cgroup_move_task().

Link: sched-ext/scx#370
Fixes: 8195136 ("sched_ext: Add cgroup support")
Cc: [email protected] # v6.12+
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: htejun

kernel/sched/autogroup.c

@@ -150,7 +150,7 @@ void sched_autogroup_exit_task(struct task_struct *p)
 	 * see this thread after that: we can no longer use signal->autogroup.
 	 * See the PF_EXITING check in task_wants_autogroup().
 	 */
-	sched_move_task(p);
+	sched_move_task(p, true);
 }
 
 static void
@@ -182,7 +182,7 @@ autogroup_move_group(struct task_struct *p, struct autogroup *ag)
 	 * sched_autogroup_exit_task().
 	 */
 	for_each_thread(p, t)
-		sched_move_task(t);
+		sched_move_task(t, true);
 
 	unlock_task_sighand(p, &flags);
 	autogroup_kref_put(prev);

kernel/sched/core.c

@@ -9042,7 +9042,7 @@ static void sched_change_group(struct task_struct *tsk, struct task_group *group
  * now. This function just updates tsk->se.cfs_rq and tsk->se.parent to reflect
  * its new group.
  */
-void sched_move_task(struct task_struct *tsk)
+void sched_move_task(struct task_struct *tsk, bool for_autogroup)
 {
 	int queued, running, queue_flags =
 		DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
@@ -9071,7 +9071,8 @@ void sched_move_task(struct task_struct *tsk)
 		put_prev_task(rq, tsk);
 
 	sched_change_group(tsk, group);
-	scx_move_task(tsk);
+	if (!for_autogroup)
+		scx_cgroup_move_task(tsk);
 
 	if (queued)
 		enqueue_task(rq, tsk, queue_flags);
@@ -9172,7 +9173,7 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset)
 	struct cgroup_subsys_state *css;
 
 	cgroup_taskset_for_each(task, css, tset)
-		sched_move_task(task);
+		sched_move_task(task, false);
 
 	scx_cgroup_finish_attach();
 }

kernel/sched/ext.c

@@ -4323,24 +4323,11 @@ int scx_cgroup_can_attach(struct cgroup_taskset *tset)
 	return ops_sanitize_err("cgroup_prep_move", ret);
 }
 
-void scx_move_task(struct task_struct *p)
+void scx_cgroup_move_task(struct task_struct *p)
 {
 	if (!scx_cgroup_enabled)
 		return;
 
-	/*
-	 * We're called from sched_move_task() which handles both cgroup and
-	 * autogroup moves. Ignore the latter.
-	 *
-	 * Also ignore exiting tasks, because in the exit path tasks transition
-	 * from the autogroup to the root group, so task_group_is_autogroup()
-	 * alone isn't able to catch exiting autogroup tasks. This is safe for
-	 * cgroup_move(), because cgroup migrations never happen for PF_EXITING
-	 * tasks.
-	 */
-	if (task_group_is_autogroup(task_group(p)) || (p->flags & PF_EXITING))
-		return;
-
 	/*
 	 * @p must have ops.cgroup_prep_move() called on it and thus
 	 * cgrp_moving_from set.

kernel/sched/ext.h

@@ -73,7 +73,7 @@ static inline void scx_update_idle(struct rq *rq, bool idle, bool do_notify) {}
 int scx_tg_online(struct task_group *tg);
 void scx_tg_offline(struct task_group *tg);
 int scx_cgroup_can_attach(struct cgroup_taskset *tset);
-void scx_move_task(struct task_struct *p);
+void scx_cgroup_move_task(struct task_struct *p);
 void scx_cgroup_finish_attach(void);
 void scx_cgroup_cancel_attach(struct cgroup_taskset *tset);
 void scx_group_set_weight(struct task_group *tg, unsigned long cgrp_weight);
@@ -82,7 +82,7 @@ void scx_group_set_idle(struct task_group *tg, bool idle);
 static inline int scx_tg_online(struct task_group *tg) { return 0; }
 static inline void scx_tg_offline(struct task_group *tg) {}
 static inline int scx_cgroup_can_attach(struct cgroup_taskset *tset) { return 0; }
-static inline void scx_move_task(struct task_struct *p) {}
+static inline void scx_cgroup_move_task(struct task_struct *p) {}
 static inline void scx_cgroup_finish_attach(void) {}
 static inline void scx_cgroup_cancel_attach(struct cgroup_taskset *tset) {}
 static inline void scx_group_set_weight(struct task_group *tg, unsigned long cgrp_weight) {}

kernel/sched/sched.h

@@ -572,7 +572,7 @@ extern void sched_online_group(struct task_group *tg,
 extern void sched_destroy_group(struct task_group *tg);
 extern void sched_release_group(struct task_group *tg);
 
-extern void sched_move_task(struct task_struct *tsk);
+extern void sched_move_task(struct task_struct *tsk, bool for_autogroup);
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 extern int sched_group_set_shares(struct task_group *tg, unsigned long shares);