Re: [PATCH 3/3] watchdog: add lockup_sys_info sysctl to dump sys info on system lockup

[Petr Mladek <pmladek@suse.com>]

On Thu 2025-11-06 10:30:32, Feng Tang wrote:
> When soft/hard lockup happens, developers may need different kinds of
> system information (call-stacks, memory info, locks, etc.) to help debugging.
> 
> Add 'lockup_sys_info' sysctl knob to take human readable string like
> "tasks,mem,timers,locks,ftrace,...", and when system lockup happens, all
> requested information will be dumped. (refer kernel/sys_info.c for more
> details).
> 
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -53,6 +54,13 @@ static int __read_mostly watchdog_hardlockup_available;
>  struct cpumask watchdog_cpumask __read_mostly;
>  unsigned long *watchdog_cpumask_bits = cpumask_bits(&watchdog_cpumask);
>  
> +/*
> + * A bitmask to control what kinds of system info to be printed when
> + * system lockup is detected, it could be task, memory, lock etc. Refer
> + * include/linux/sys_info.h for detailed bit definition.
> + */
> +static unsigned long lockup_si_mask;
> +
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>  
>  # ifdef CONFIG_SMP
> @@ -240,6 +248,7 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
>  				clear_bit_unlock(0, &hard_lockup_nmi_warn);
>  		}

The code right above printed backtaces from all CPUs when
sysctl_hardlockup_all_cpu_backtrace.

> +		sys_info(lockup_si_mask);

And sys_info() could print it again when SYS_INFO_ALL_BT
bit is set. The hard lockup detector should use the same
trick as the softlockup detector in watchdog_timer_fn().

>  		if (hardlockup_panic)
>  			nmi_panic(regs, "Hard LOCKUP");
>  
> @@ -746,9 +755,11 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  	unsigned long touch_ts, period_ts, now;
>  	struct pt_regs *regs = get_irq_regs();
>  	int duration;
> -	int softlockup_all_cpu_backtrace = sysctl_softlockup_all_cpu_backtrace;
> +	int softlockup_all_cpu_backtrace;
>  	unsigned long flags;
>  
> +	softlockup_all_cpu_backtrace = (lockup_si_mask & SYS_INFO_ALL_BT) ?
> +					1 : sysctl_softlockup_all_cpu_backtrace;
>  	if (!watchdog_enabled)
>  		return HRTIMER_NORESTART;
>  
> @@ -846,6 +857,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  		}
>  
>  		add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
> +		sys_info(lockup_si_mask & ~SYS_INFO_ALL_BT);
>  		if (softlockup_panic)
>  			panic("softlockup: hung tasks");
>  	}
> @@ -1178,6 +1190,13 @@ static const struct ctl_table watchdog_sysctls[] = {
>  		.mode		= 0644,
>  		.proc_handler	= proc_watchdog_cpumask,
>  	},
> +	{
> +		.procname	= "lockup_sys_info",
> +		.data		= &lockup_si_mask,
> +		.maxlen         = sizeof(lockup_si_mask),
> +		.mode		= 0644,
> +		.proc_handler	= sysctl_sys_info_handler,
> +	},

There already exists:

	+ hardlockup_all_cpu_backtrace
	+ hardlockup_panic
	+ softlockup_all_cpu_backtrace
	+ softlockup_panic

IMHO, it would make sense to introduce separate:

	+ hardlockup_sys_info
	+ softlockup_sys_info


Best Regards,
Petr