sched_ext: Merge branch 'for-6.17-fixes' into for-6.18

Pull sched_ext/for-6.17-fixes to receive:

 55ed11b181 ("sched_ext: idle: Handle migration-disabled tasks in BPF code")

which conflicts with the following commit in for-6.18:

 2407bae23d ("sched_ext: Add the @sch parameter to ext_idle helpers")

The conflict is a simple context conflict which can be resolved by taking
the updated parts from both commits.

Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/sched/core.c

@@ -9549,7 +9549,7 @@ static unsigned long tg_weight(struct task_group *tg)
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	return scale_load_down(tg->shares);
 #else
-	return sched_weight_from_cgroup(tg->scx_weight);
+	return sched_weight_from_cgroup(tg->scx.weight);
 #endif
 }
 

kernel/sched/ext.c

@@ -4662,6 +4662,9 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 			__setscheduler_class(p->policy, p->prio);
 		struct sched_enq_and_set_ctx ctx;
 
+		if (!tryget_task_struct(p))
+			continue;
+
 		if (old_class != new_class && p->se.sched_delayed)
 			dequeue_task(task_rq(p), p, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
 
@@ -4674,6 +4677,7 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 		sched_enq_and_set_task(&ctx);
 
 		check_class_changed(task_rq(p), p, old_class, p->prio);
+		put_task_struct(p);
 	}
 	scx_task_iter_stop(&sti);
 	percpu_up_write(&scx_fork_rwsem);
@@ -5740,12 +5744,8 @@ __bpf_kfunc u32 scx_bpf_reenqueue_local(void)
 		 * CPUs disagree, they use %ENQUEUE_RESTORE which is bypassed to
 		 * the current local DSQ for running tasks and thus are not
 		 * visible to the BPF scheduler.
-		 *
-		 * Also skip re-enqueueing tasks that can only run on this
-		 * CPU, as they would just be re-added to the same local
-		 * DSQ without any benefit.
 		 */
-		if (p->migration_pending || is_migration_disabled(p) || p->nr_cpus_allowed == 1)
+		if (p->migration_pending)
 			continue;
 
 		dispatch_dequeue(rq, p);

kernel/sched/ext_idle.c

@@ -856,6 +856,32 @@ static bool check_builtin_idle_enabled(struct scx_sched *sch)
 	return false;
 }
 
+/*
+ * Determine whether @p is a migration-disabled task in the context of BPF
+ * code.
+ *
+ * We can't simply check whether @p->migration_disabled is set in a
+ * sched_ext callback, because migration is always disabled for the current
+ * task while running BPF code.
+ *
+ * The prolog (__bpf_prog_enter) and epilog (__bpf_prog_exit) respectively
+ * disable and re-enable migration. For this reason, the current task
+ * inside a sched_ext callback is always a migration-disabled task.
+ *
+ * Therefore, when @p->migration_disabled == 1, check whether @p is the
+ * current task or not: if it is, then migration was not disabled before
+ * entering the callback, otherwise migration was disabled.
+ *
+ * Returns true if @p is migration-disabled, false otherwise.
+ */
+static bool is_bpf_migration_disabled(const struct task_struct *p)
+{
+	if (p->migration_disabled == 1)
+		return p != current;
+	else
+		return p->migration_disabled;
+}
+
 static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p,
 				 s32 prev_cpu, u64 wake_flags,
 				 const struct cpumask *allowed, u64 flags)
@@ -899,7 +925,7 @@ static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p,
 	 * selection optimizations and simply check whether the previously
 	 * used CPU is idle and within the allowed cpumask.
 	 */
-	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
+	if (p->nr_cpus_allowed == 1 || is_bpf_migration_disabled(p)) {
 		if (cpumask_test_cpu(prev_cpu, allowed ?: p->cpus_ptr) &&
 		    scx_idle_test_and_clear_cpu(prev_cpu))
 			cpu = prev_cpu;

tools/testing/selftests/sched_ext/hotplug.c

@@ -6,7 +6,6 @@
 #include <bpf/bpf.h>
 #include <sched.h>
 #include <scx/common.h>
-#include <sched.h>
 #include <sys/wait.h>
 #include <unistd.h>