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

[ I resend and fix my reply, my previous reply seems to have an
issue with the "Date" ]

Hi Zqiang,

On Mon, 24 Jan 2022 18:36:37 +0800, Zqiang <qiang1.zhang@intel.com> wrote:> +static void rcuc_kthread_dump(struct rcu_data *rdp)
> +{
> + int cpu;
> + unsigned long j;
> + struct task_struct *rcuc = rdp->rcu_cpu_kthread_task;
> +
> + if (rcu_is_rcuc_kthread_starving(rdp, &j)) {
> + cpu = rcuc ? task_cpu(rcuc) : -1;
> +
> + if (rcuc) {
> + pr_err("%s kthread starved for %ld jiffies, stack dump:\n",
> + rcuc->comm, j);
> + sched_show_task(rcuc);
> + if (cpu >= 0) {
> + if (cpu_online(cpu) && !idle_cpu(cpu)) {
> + pr_err("Dump current CPU stack:\n");
> + if (!trigger_single_cpu_backtrace(cpu))
> + dump_cpu_task(cpu);
> + }
> + }
> + }
> + }
> +}

1) We can reduce the nested if with an early return after
checking `rcu_is_rcuc_kthread_starving()`.

2) This ternary operator doesn't make sense:

`cpu = rcuc ? task_cpu(rcuc) : -1;`

If `rcuc` is NULL, then the "if (rcuc)" block will never
be executed, and `cpu` variable won't be used, why should
we perform a conditional with ternary to assign -1 here?

3) We can use an early return as well for the `if (rcuc)` to
avoid more nested if.

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);
sched_show_task(rcuc);
cpu = task_cpu(rcuc);
if (cpu_online(cpu) && !idle_cpu(cpu)) {
pr_err("Dump current CPU stack:\n");
if (!trigger_single_cpu_backtrace(cpu))
dump_cpu_task(cpu);
}
}
```

Thank you!

--
Ammar Faizi

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