Re: [PATCH v5 15/18] watchdog/perf: Add a weak function for an arch to detect if perf can use NMIs

[Petr Mladek]

On Fri 2023-05-19 10:18:39, Douglas Anderson wrote:
> On arm64, NMI support needs to be detected at runtime. Add a weak
> function to the perf hardlockup detector so that an architecture can
> implement it to detect whether NMIs are available.
> 
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> While I won't object to this patch landing, I consider it part of the
> arm64 perf hardlockup effort. I would be OK with the earlier patches
> in the series landing and then not landing ${SUBJECT} patch nor
> anything else later.
> 
> I'll also note that, as an alternative to this, it would be nice if we
> could figure out how to make perf_event_create_kernel_counter() fail
> on arm64 if NMIs aren't available. Maybe we could add a "must_use_nmi"
> element to "struct perf_event_attr"?
> 
> --- a/kernel/watchdog_perf.c
> +++ b/kernel/watchdog_perf.c
> @@ -234,12 +234,22 @@ void __init hardlockup_detector_perf_restart(void)
>  	}
>  }
>  
> +bool __weak __init arch_perf_nmi_is_available(void)
> +{
> +	return true;
> +}
> +
>  /**
>   * watchdog_hardlockup_probe - Probe whether NMI event is available at all
>   */
>  int __init watchdog_hardlockup_probe(void)
>  {
> -	int ret = hardlockup_detector_event_create();
> +	int ret;
> +
> +	if (!arch_perf_nmi_is_available())
> +		return -ENODEV;

My understanding is that this would block the perf hardlockup detector
at runtime. Does it work with the "nmi_watchdog" sysctl. I see
that it is made read-only when it is not enabled at build time,
see NMI_WATCHDOG_SYSCTL_PERM.

> +
> +	ret = hardlockup_detector_event_create();
>  
>  	if (ret) {
>  		pr_info("Perf NMI watchdog permanently disabled\n");

Best Regards,
Petr