linux-kernel - Re: [PATCH v3 07/10] sched/psi: add PSI

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Date:   Wed, 24 Aug 2022 19:53:52 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     tj@...nel.org, mkoutny@...e.com, surenb@...gle.com,
        gregkh@...uxfoundation.org, corbet@....net, mingo@...hat.com,
        peterz@...radead.org, songmuchun@...edance.com,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 07/10] sched/psi: add PSI_IRQ to track IRQ/SOFTIRQ
 pressure

On 2022/8/24 18:46, Johannes Weiner wrote:
> On Wed, Aug 24, 2022 at 04:18:26PM +0800, Chengming Zhou wrote:
>> @@ -903,6 +903,36 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
>>  	}
>>  }
>>  
>> +#ifdef CONFIG_IRQ_TIME_ACCOUNTING
>> +void psi_account_irqtime(struct task_struct *task, u32 delta)
>> +{
>> +	int cpu = task_cpu(task);
>> +	void *iter = NULL;
>> +	struct psi_group *group;
>> +	struct psi_group_cpu *groupc;
>> +	u64 now;
>> +
>> +	if (!task->pid)
>> +		return;
>> +
>> +	now = cpu_clock(cpu);
>> +
>> +	while ((group = iterate_groups(task, &iter))) {
>> +		groupc = per_cpu_ptr(group->pcpu, cpu);
>> +
>> +		write_seqcount_begin(&groupc->seq);
>> +
>> +		record_times(groupc, now);
>> +		groupc->times[PSI_IRQ_FULL] += delta;
>> +
>> +		write_seqcount_end(&groupc->seq);
>> +
>> +		if (group->poll_states & (1 << PSI_IRQ_FULL))
>> +			psi_schedule_poll_work(group, 1);
>> +	}
> 
> Shouldn't this kick avgs_work too? If the CPU is otherwise idle,
> times[PSI_IRQ_FULL] would overflow after two missed averaging runs.

If the CPU is idle, task->pid == 0, so no times[PSI_IRQ_FULL] would accumulate?
I was thinking if task->pid != 0, avgs_work should be active.

Not sure, maybe I missed something.

> 
> avgs_work should probably also self-perpetuate when PSI_IRQ_FULL is in
> changed_states. (Looking at that code, I think it can be simplified:
> delete nonidle and do `if (changed_states) schedule_delayed_work()`.)

```
collect_percpu_times(group, PSI_AVGS, &changed_states);
nonidle = changed_states & (1 << PSI_NONIDLE);

if (nonidle) {
	schedule_delayed_work(dwork, nsecs_to_jiffies(
			group->avg_next_update - now) + 1);
}
```

Yes, changed_states include PSI_IRQ_FULL, here we only check nonidle
= changed_states & (1 << PSI_NONIDLE), so it will not restart
if only PSI_IRQ_FULL?

If use `if (changed_states) schedule_delayed_work()`, avgs_work will
self-restart when only PSI_IRQ_FULL changes?

Thanks!