[PATCH RFC v2 15/15] rcu-tasks: Kick running holdouts through the scheduler

A holdout that is running on a CPU with nothing else runnable is only
preempted if the tick acts on rcu_request_urgent_qs_task()'s flag, and
there may be no tick.  Now that a preemption outside a trampoline is a
quiescent state, have check_holdout_task() call resched_cpu() on a
running holdout as well, so the scheduler IPIs it and it goes through
__schedule() and reports (or, if it is inside a trampoline, does not
report) its own state with purely local ordering.  Nothing reads a
running task's rcu_tramp_nesting remotely.

Only under CONFIG_RCU_TASKS_PREEMPT_QS; other architectures are
unchanged.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 kernel/rcu/tasks.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index ba432bd922e2..02d2592ab7a3 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h

@@ -1038,8 +1038,18 @@ static bool rcu_tasks_preempted_qs(struct task_struct *t)
 {
 	return task_call_func(t, rcu_tasks_switched_out_clean, NULL);
 }
+
+/* Make a running holdout pass through __schedule() soon, tick or no tick. */
+static void rcu_tasks_kick_running(struct task_struct *t)
+{
+	int cpu = task_cpu(t);
+
+	if (task_curr(t) && cpu_online(cpu))
+		resched_cpu(cpu);
+}
 #else
 static bool rcu_tasks_preempted_qs(struct task_struct *t) { return false; }
+static void rcu_tasks_kick_running(struct task_struct *t) { }
 #endif
 
 /* Per-task initial processing. */

@@ -1219,6 +1229,7 @@ static void check_holdout_task(struct task_struct *t,
 		return;
 	}
 	rcu_request_urgent_qs_task(t);
+	rcu_tasks_kick_running(t);
 	if (!needreport)
 		return;
 	if (*firstreport) {

-- 
2.55.0