Re: [PATCH v2] rcu: Add per-CPU rcuc task info to RCU CPU stall warnings

[Ammar Faizi]

On 1/24/22 11:42 PM, Paul E. McKenney wrote:
> On Mon, Jan 24, 2022 at 05:38:21PM +0700, Ammar Faizi wrote:
> > [snip...]
> > FWIW, this one makes more sense:
> > ```
> > static void rcuc_kthread_dump(struct rcu_data *rdp)
> > {
> > 	 int cpu;
> > 	 unsigned long j;
> > 	 struct task_struct *rcuc;
> >
> > 	 if (!rcu_is_rcuc_kthread_starving(rdp, &j))
> > 		 return;
> >
> > 	 rcuc = rdp->rcu_cpu_kthread_task;
> > 	 if (!rcuc)
> > 		 return;
> >
> > 	 pr_err("%s kthread starved for %ld jiffies, stack dump:\n", rcuc->comm, j);
>
> Thank you for looking this over and for the great feedback, Ammar!
>
> I am also wondering why the above message should be printed when the
> corresponding CPU is offline or idle.  Why not move the above pr_err()
> line down to replace the pr_err() line below?
>
> 							Thanx, Paul

Hi Paul, Thank you for the review. Agree with that.
Hopefully this one looks better (untested):
```
static void rcuc_kthread_dump(struct rcu_data *rdp)
{
	int cpu;
	unsigned long j;
	struct task_struct *rcuc;

	rcuc = rdp->rcu_cpu_kthread_task;
	if (!rcuc)
		return;

	cpu = task_cpu(rcuc);
	if (cpu_is_offline(cpu) || idle_cpu(cpu))
		return;

	if (!rcu_is_rcuc_kthread_starving(rdp, &j))
		return;

	pr_err("%s kthread starved for %ld jiffies\n", rcuc->comm, j);
	sched_show_task(rcuc);
	if (!trigger_single_cpu_backtrace(cpu))
		dump_cpu_task(cpu);
}
```

Recall that dump_cpu_task looks like this:
```
void dump_cpu_task(int cpu)
{
	pr_info("Task dump for CPU %d:\n", cpu);
	sched_show_task(cpu_curr(cpu));
}
```
which already tells us it's a dump, so "stack dump" in the pr_err()
can be omitted. Any comment, Zqiang?

--
Ammar Faizi