sched_ext: Fixes for v7.1-rc2

- Fix idle CPU selection returning prev_cpu outside the task's cpus_ptr
   when the BPF caller's allowed mask was wider. Stable backport.
 
 - Two opposite-direction gaps in scx_task_iter's cgroup-scoped mode
   versus the global mode:
 
   - Tasks past exit_signals() are filtered by the cgroup walk but kept by
     global. Sub-scheduler enable abort leaked __scx_init_task() state.
     Add a CSS_TASK_ITER_WITH_DEAD flag to cgroup's task iterator
     (scx_task_iter is its only user) and use it.
 
   - Tasks past sched_ext_dead() are still returned, tripping
     WARN_ON_ONCE() in callers or making them touch torn-down state. Mark
     and skip under the per-task rq lock.
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Merge tag 'sched_ext-for-7.1-rc2-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext

Pull sched_ext fixes from Tejun Heo:

 - Fix idle CPU selection returning prev_cpu outside the task's cpus_ptr
   when the BPF caller's allowed mask was wider. Stable backport.

 - Two opposite-direction gaps in scx_task_iter's cgroup-scoped mode
   versus the global mode:

    - Tasks past exit_signals() are filtered by the cgroup walk but kept
      by global. Sub-scheduler enable abort leaked __scx_init_task()
      state. Add a CSS_TASK_ITER_WITH_DEAD flag to cgroup's task
      iterator (scx_task_iter is its only user) and use it.

    - Tasks past sched_ext_dead() are still returned, tripping
      WARN_ON_ONCE() in callers or making them touch torn-down state.
      Mark and skip under the per-task rq lock.

* tag 'sched_ext-for-7.1-rc2-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext:
  sched_ext: idle: Recheck prev_cpu after narrowing allowed mask
  sched_ext: Skip past-sched_ext_dead() tasks in scx_task_iter_next_locked()
  cgroup, sched_ext: Include exiting tasks in cgroup iter

include/linux/cgroup.h

@@ -53,6 +53,7 @@ struct kernel_clone_args;
 enum css_task_iter_flags {
 	CSS_TASK_ITER_PROCS    = (1U << 0),  /* walk only threadgroup leaders */
 	CSS_TASK_ITER_THREADED = (1U << 1),  /* walk all threaded css_sets in the domain */
+	CSS_TASK_ITER_WITH_DEAD = (1U << 2),  /* include exiting tasks */
 	CSS_TASK_ITER_SKIPPED  = (1U << 16), /* internal flags */
 };
 

include/linux/sched/ext.h

@@ -101,6 +101,7 @@ enum scx_ent_flags {
 	SCX_TASK_DEQD_FOR_SLEEP	= 1 << 3, /* last dequeue was for SLEEP */
 	SCX_TASK_SUB_INIT	= 1 << 4, /* task being initialized for a sub sched */
 	SCX_TASK_IMMED		= 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
+	SCX_TASK_OFF_TASKS	= 1 << 6, /* removed from scx_tasks by sched_ext_dead() */
 
 	/*
 	 * Bits 8 and 9 are used to carry task state:

kernel/cgroup/cgroup.c

@@ -5067,10 +5067,12 @@ repeat:
 
 	task = list_entry(it->task_pos, struct task_struct, cg_list);
 	/*
-	 * Hide tasks that are exiting but not yet removed. Keep zombie
-	 * leaders with live threads visible.
+	 * Hide tasks that are exiting but not yet removed by default. Keep
+	 * zombie leaders with live threads visible. Usages that need to walk
+	 * every existing task can opt out via CSS_TASK_ITER_WITH_DEAD.
 	 */
-	if ((task->flags & PF_EXITING) && !atomic_read(&task->signal->live))
+	if (!(it->flags & CSS_TASK_ITER_WITH_DEAD) &&
+	    (task->flags & PF_EXITING) && !atomic_read(&task->signal->live))
 		goto repeat;
 
 	if (it->flags & CSS_TASK_ITER_PROCS) {

kernel/sched/ext.c

@@ -766,7 +766,8 @@ static void scx_task_iter_start(struct scx_task_iter *iter, struct cgroup *cgrp)
 		lockdep_assert_held(&cgroup_mutex);
 		iter->cgrp = cgrp;
 		iter->css_pos = css_next_descendant_pre(NULL, &iter->cgrp->self);
-		css_task_iter_start(iter->css_pos, 0, &iter->css_iter);
+		css_task_iter_start(iter->css_pos, CSS_TASK_ITER_WITH_DEAD,
+				    &iter->css_iter);
 		return;
 	}
 #endif
@@ -866,7 +867,8 @@ static struct task_struct *scx_task_iter_next(struct scx_task_iter *iter)
 			iter->css_pos = css_next_descendant_pre(iter->css_pos,
 								&iter->cgrp->self);
 			if (iter->css_pos)
-				css_task_iter_start(iter->css_pos, 0, &iter->css_iter);
+				css_task_iter_start(iter->css_pos, CSS_TASK_ITER_WITH_DEAD,
+						    &iter->css_iter);
 		}
 		return NULL;
 	}
@@ -926,16 +928,27 @@ static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
 		 *
 		 * Test for idle_sched_class as only init_tasks are on it.
 		 */
-		if (p->sched_class != &idle_sched_class)
-			break;
+		if (p->sched_class == &idle_sched_class)
+			continue;
+
+		iter->rq = task_rq_lock(p, &iter->rf);
+		iter->locked_task = p;
+
+		/*
+		 * cgroup_task_dead() removes the dead tasks from cset->tasks
+		 * after sched_ext_dead() and cgroup iteration may see tasks
+		 * which already finished sched_ext_dead(). %SCX_TASK_OFF_TASKS
+		 * is set by sched_ext_dead() under @p's rq lock. Test it to
+		 * avoid visiting tasks which are already dead from SCX POV.
+		 */
+		if (p->scx.flags & SCX_TASK_OFF_TASKS) {
+			__scx_task_iter_rq_unlock(iter);
+			continue;
+		}
+
+		return p;
 	}
-	if (!p)
-		return NULL;
-
-	iter->rq = task_rq_lock(p, &iter->rf);
-	iter->locked_task = p;
-
-	return p;
+	return NULL;
 }
 
 /**
@@ -3848,6 +3861,11 @@ void sched_ext_dead(struct task_struct *p)
 	/*
 	 * @p is off scx_tasks and wholly ours. scx_root_enable()'s READY ->
 	 * ENABLED transitions can't race us. Disable ops for @p.
+	 *
+	 * %SCX_TASK_OFF_TASKS synchronizes against cgroup task iteration - see
+	 * scx_task_iter_next_locked(). NONE tasks need no marking: cgroup
+	 * iteration is only used from sub-sched paths, which require root
+	 * enabled. Root enable transitions every live task to at least READY.
 	 */
 	if (scx_get_task_state(p) != SCX_TASK_NONE) {
 		struct rq_flags rf;
@@ -3855,6 +3873,7 @@ void sched_ext_dead(struct task_struct *p)
 
 		rq = task_rq_lock(p, &rf);
 		scx_disable_and_exit_task(scx_task_sched(p), p);
+		p->scx.flags |= SCX_TASK_OFF_TASKS;
 		task_rq_unlock(rq, p, &rf);
 	}
 }

kernel/sched/ext_idle.c

@@ -465,12 +465,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 
 	preempt_disable();
 
-	/*
-	 * Check whether @prev_cpu is still within the allowed set. If not,
-	 * we can still try selecting a nearby CPU.
-	 */
-	is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
-
 	/*
 	 * Determine the subset of CPUs usable by @p within @cpus_allowed.
 	 */
@@ -487,6 +481,12 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 		}
 	}
 
+	/*
+	 * Check whether @prev_cpu is still within the allowed set. If not,
+	 * we can still try selecting a nearby CPU.
+	 */
+	is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
+
 	/*
 	 * This is necessary to protect llc_cpus.
 	 */