Re: [PATCH v2 1/2] sched/core: Fixed missing rq clock update before calling set_rq_offline()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 10, 2023 at 04:34:49PM +0800, Hao Jia wrote:
> This is triggered during cpu offline when CONFIG_CPU_FREQ is enabled:

I'm failing to see the relevance to CPU_FREQ...

> ------------[ 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

AFAICT this is __disable_runtime() re-queueing throttled RT tasks

Your unwind is dodgy, what CONFIG_UNWINDER do you have?

>  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
> 
> Before calling set_rq_offline() we need to update the rq clock to avoid
> using the old rq clock, and use rq_lock_irqsave()/rq_unlock_irqrestore()
> to replace raw_spin_rq_lock_irqsave()/raw_spin_rq_unlock_irqrestore() 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
> 
> Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
> ---
>  kernel/sched/topology.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 6682535e37c8..b89497696880 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -487,15 +487,17 @@ 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;
>  
> -		if (cpumask_test_cpu(rq->cpu, old_rd->online))
> +		if (cpumask_test_cpu(rq->cpu, old_rd->online)) {
> +			update_rq_clock(rq);
>  			set_rq_offline(rq);
> +		}
>  
>  		cpumask_clear_cpu(rq->cpu, old_rd->span);
>  
> @@ -515,7 +517,7 @@ 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);

The patch itself is good though; just the Changelog has me
confused.