From 777570d9ef820e470736fa9e02b8e3e48891c050 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Tue, 8 Mar 2022 09:54:13 -0800 Subject: [PATCH] rcu-tasks: Use schedule_hrtimeout_range() to wait for grace periods The synchronous RCU-tasks grace-period-wait primitives invoke schedule_timeout_idle() to give readers a chance to exit their read-side critical sections. Unfortunately, this fails during early boot on PREEMPT_RT because PREEMPT_RT relies solely on ksoftirqd to run timer handlers. Because ksoftirqd cannot operate until its kthreads are spawned, there is a brief period of time following scheduler initialization where PREEMPT_RT cannot run the timer handlers that schedule_timeout_idle() relies on, resulting in a hang. To avoid this boot-time hang, this commit replaces schedule_timeout_idle() with schedule_hrtimeout(), so that the timer expires in hardirq context. This is ensures that the timer fires even on PREEMPT_RT throughout the irqs-enabled portions of boot as well as during runtime. The timer is set to expire between fract and fract + HZ / 2 jiffies in order to align with any other timers that might expire during that time, thus reducing the number of wakeups. Note that RCU-tasks grace periods are infrequent, so the use of hrtimer should be fine. In contrast, in common-case code, user of hrtimer could result in performance issues. Cc: Martin KaFai Lau Cc: Andrii Nakryiko Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Paul E. McKenney --- kernel/rcu/tasks.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 4b91cb214ca7..71fe340ab82a 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -647,13 +647,16 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp) fract = rtp->init_fract; while (!list_empty(&holdouts)) { + ktime_t exp; bool firstreport; bool needreport; int rtst; // Slowly back off waiting for holdouts set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS); - schedule_timeout_idle(fract); + exp = jiffies_to_nsecs(fract); + __set_current_state(TASK_IDLE); + schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD); if (fract < HZ) fract++;