Re: [PATCH] lib/nmi_backtrace: print out the CPUs which fail to respond to NMI

[Andrew Morton]

On Thu, 21 May 2026 11:03:36 +0800 Feng Tang <feng.tang@xxxxxxxxxxxxxxxxx> wrote:

> When debugging RCU stall cases, usually all CPUs will respond to the
> NMI and print out the backtrace. But in some nasty or hardware related
> cases, some CPUs may fail to respond in 10 seconds, and very likely
> this is sign of severe issues.
> 
> Paul E. McKenney has implemented the NMI backtrace stall check for x86,
> and for other architectures, it should be also helpful to at least
> print out those CPUs which failed to repond to the NMI, so that users
> can get an early heads-up for possible CPU hard stall.

That must be one messed up machine.  Is this something you've
encountered in real life?

> --- a/lib/nmi_backtrace.c
> +++ b/lib/nmi_backtrace.c
> @@ -75,7 +75,13 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
>  		mdelay(1);
>  		touch_softlockup_watchdog();
>  	}
> -	nmi_backtrace_stall_check(to_cpumask(backtrace_mask));
> +
> +	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
> +		pr_warn("After 10 seconds, these CPUS still haven't responded to the NMI: %*pbl\n",
> +			cpumask_pr_args(to_cpumask(backtrace_mask)));
> +
> +		nmi_backtrace_stall_check(to_cpumask(backtrace_mask));
> +	}

It's a nitpick, but

: 	/* Wait for up to 10 seconds for all CPUs to do the backtrace */
: 	for (i = 0; i < 10 * 1000; i++) {
: 		if (cpumask_empty(to_cpumask(backtrace_mask)))
: 			break;
: 		mdelay(1);
: 		touch_softlockup_watchdog();
: 	}
: 
: 	if (!cpumask_empty(to_cpumask(backtrace_mask))) {
: 		pr_warn("After 10 seconds, these CPUS still haven't responded to the NMI: %*pbl\n",

Here we're hard-coding "10" in two places and in a comment.  It would
be nicer to do

	#define FOO_TIMEOUT 10

then use that throughout.

(bonus points for figuring out how to paste that "10" into the
pr_warn() control string rather than using %d!)