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

[Hao Jia <jiahao.os@bytedance.com>]

On 2023/6/13 Hao Jia wrote:
> 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@google.com>
> Signed-off-by: Hao Jia <jiahao.os@bytedance.com>

Hi Vincent,

   Benjamin Segall suggested adding update_rq_clock() to set_rq_offline()
instead of calling update_rq_clock() before set_rq_offline() runs.
The code of patch1 has changed, so I removed your tag "Reviewed-by". 
Please review again.

Thanks,
Hao

> ---
>   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);