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

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Date:   Tue, 25 Jan 2022 02:24:47 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] rcu: Add per-CPU rcuc task info to RCU CPU stall
 warnings


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?

Thanks Ammar, this look like more compact, I wiil resend.

Thanks
Zqiang

>>>>
>>--
>>Ammar Faizi