[PATCH v5 1/4] sched/core: Fixed missing rq clock update before calling set_rq_offline()

[Hao Jia]

This is triggered during cpu offline when CONFIG_CPU_FREQ is enabled
and cpufreq is set to powersave:
------------[ cut here ]------------
rq->clock_update_flags < RQCF_ACT_SKIP
WARNING: CPU: 24 PID: 754 at kernel/sched/sched.h:1496
enqueue_top_rt_rq+0x139/0x160
Call Trace:
 <TASK>
 ? intel_pstate_update_util+0x3b0/0x3b0
 rq_offline_rt+0x1b7/0x250
 set_rq_offline.part.120+0x28/0x60
 rq_attach_root+0xc4/0xd0
 cpu_attach_domain+0x3dc/0x7f0
 ? __schedule+0x65e/0x1310
 partition_sched_domains_locked+0x2a5/0x3c0
 rebuild_sched_domains_locked+0x477/0x830
 ? percpu_rwsem_wait+0x140/0x140
 rebuild_sched_domains+0x1b/0x30
 cpuset_hotplug_workfn+0x2ca/0xc90
 ? balance_push+0x56/0x120
 ? _raw_spin_unlock+0x15/0x30
 ? finish_task_switch+0x98/0x2f0
 ? __switch_to+0x116/0x410
 ? __schedule+0x65e/0x1310 ? internal_add_timer+0x42/0x60
 ? _raw_spin_unlock_irqrestore+0x23/0x40
 ? add_timer_on+0xd5/0x130
 process_one_work+0x1bc/0x3d0
 worker_thread+0x4c/0x380
 ? preempt_count_add+0x56/0xa0
 ? rescuer_thread+0x310/0x310
 kthread+0xe6/0x110
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork+0x1f/0x30

More detailed key function call graph:
rq_offline_rt()
  __disable_runtime()
    sched_rt_rq_enqueue()
      enqueue_top_rt_rq()
        cpufreq_update_util() <-- depends on CONFIG_CPU_FREQ
          data->func(data, *rq_clock(rq)*, flags)
            intel_pstate_update_util() <-- powersave policy callback function

Before calling rq_offline_rt() we need to update the rq clock to avoid
using the old rq clock, So we add update_rq_clock() to set_rq_offline()
to update rq clock. And we use rq_lock_irqsave()/rq_unlock_irqrestore()
to replace raw_spin_rq_lock_irqsave()/raw_spin_rq_unlock_irqrestore()
in rq_attach_root() to ensure that rq->clock_update_flags are cleared
before updating the rq clock.

Steps to reproduce:
1. Enable CONFIG_SMP and CONFIG_CPU_FREQ when compiling the kernel
2. echo 1 > /sys/kernel/debug/clear_warn_once
3. cpupower -c all frequency-set -g powersave
4. Run some rt tasks e.g. Create 5*n rt (100% running) tasks (on a
   system with n CPUs)
5. Offline cpu one by one until the warninng is triggered

Suggested-by: Ben Segall <bsegall@xxxxxxxxxx>
Signed-off-by: Hao Jia <jiahao.os@xxxxxxxxxxxxx>
---
 kernel/sched/core.c     | 2 +-
 kernel/sched/topology.c | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a68d1276bab0..a8be5415daba 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9548,6 +9548,7 @@ void set_rq_offline(struct rq *rq)
 	if (rq->online) {
 		const struct sched_class *class;
 
+		update_rq_clock(rq);
 		for_each_class(class) {
 			if (class->rq_offline)
 				class->rq_offline(rq);
@@ -9689,7 +9690,6 @@ int sched_cpu_deactivate(unsigned int cpu)
 
 	rq_lock_irqsave(rq, &rf);
 	if (rq->rd) {
-		update_rq_clock(rq);
 		BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
 		set_rq_offline(rq);
 	}
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6682535e37c8..52976eadfee9 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -487,9 +487,9 @@ static void free_rootdomain(struct rcu_head *rcu)
 void rq_attach_root(struct rq *rq, struct root_domain *rd)
 {
 	struct root_domain *old_rd = NULL;
-	unsigned long flags;
+	struct rq_flags rf;
 
-	raw_spin_rq_lock_irqsave(rq, flags);
+	rq_lock_irqsave(rq, &rf);
 
 	if (rq->rd) {
 		old_rd = rq->rd;
@@ -515,7 +515,7 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd)
 	if (cpumask_test_cpu(rq->cpu, cpu_active_mask))
 		set_rq_online(rq);
 
-	raw_spin_rq_unlock_irqrestore(rq, flags);
+	rq_unlock_irqrestore(rq, &rf);
 
 	if (old_rd)
 		call_rcu(&old_rd->rcu, free_rootdomain);
-- 
2.20.1