[PATCH 3/7] sched/core: Allow newidle for core-sched

[Peter Zijlstra <peterz@infradead.org>]

As reported [1], it is possible for newidle, as called from pick_task_fair(),
to migrate a task that was already selected for the SMT sibling.

That is, in case of core scheduling (pick_next_task()'s restart_multi label),
pick_task() is called for each SMT sibling. Suppose SMT0 picks task A, and SMT1
has no tasks and does newidle, it must not be possible (but currently is) to
migrate A to SMT1 and also select A.

Avoids the 'stealing', but doesn't yet enable newidle just yet.

[1] https://patch.msgid.link/20260603095108.GA1684319@bytedance.com

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c  |    3 ++-
 kernel/sched/fair.c  |    6 ++----
 kernel/sched/sched.h |   15 ++++++++++++++-
 3 files changed, 18 insertions(+), 6 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3049,7 +3049,8 @@ static int affine_move_task(struct rq *r
 		return -EINVAL;
 	}
 
-	if (task_on_cpu(rq, p) || READ_ONCE(p->__state) == TASK_WAKING) {
+	if (task_on_cpu(rq, p) || task_on_core(rq, p) ||
+	    READ_ONCE(p->__state) == TASK_WAKING) {
 		/*
 		 * MIGRATE_ENABLE gets here because 'p == current', but for
 		 * anything else we cannot do is_migration_disabled(), punt
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10906,7 +10903,8 @@ int can_migrate_task(struct task_struct
 	env->flags &= ~LBF_ALL_PINNED;
 
 	if (task_on_cpu(env->src_rq, p) ||
-	    task_current_donor(env->src_rq, p)) {
+	    task_current_donor(env->src_rq, p) ||
+	    task_on_core(env->src_rq, p)) {
 		schedstat_inc(p->stats.nr_failed_migrations_running);
 		return 0;
 	}
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1573,6 +1573,14 @@ static inline bool task_has_sched_core(s
 	return !!p->core_cookie;
 }
 
+static inline bool task_on_core(struct rq *rq, struct task_struct *p)
+{
+	if (sched_core_disabled())
+		return false;
+
+	return rq->core_pick == p;
+}
+
 #else /* !CONFIG_SCHED_CORE: */
 
 static inline bool sched_core_enabled(struct rq *rq)
@@ -1618,6 +1626,11 @@ static inline bool task_has_sched_core(s
 	return false;
 }
 
+static inline bool task_on_core(struct rq *rq, struct task_struct *p)
+{
+	return false;
+}
+
 #endif /* !CONFIG_SCHED_CORE */
 
 #ifdef CONFIG_RT_GROUP_SCHED
@@ -4148,7 +4161,7 @@ void move_queued_task_locked(struct rq *
 static inline
 bool task_is_pushable(struct rq *rq, struct task_struct *p, int cpu)
 {
-	if (!task_on_cpu(rq, p) &&
+	if (!task_on_cpu(rq, p) && !task_on_core(rq, p) &&
 	    cpumask_test_cpu(cpu, &p->cpus_mask))
 		return true;