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

[Mark Rutland <mark.rutland@arm.com>]

On Fri, May 19, 2023 at 10:18:39AM -0700, 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@chromium.org>
> ---
> 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.

FWIW, everything prior to this looks fine to me, so I reckon it'd be worth
splitting the series here and getting the buddy lockup detector in first, to
avoid a log-jam on all the subsequent NMI bits.

Thanks,
Mark.

> 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"?
> 
> (no changes since v4)
> 
> Changes in v4:
> - ("Add a weak function for an arch to detect ...") new for v4.
> 
>  include/linux/nmi.h    |  1 +
>  kernel/watchdog_perf.c | 12 +++++++++++-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/nmi.h b/include/linux/nmi.h
> index 47db14e7da52..eb616fc07c85 100644
> --- a/include/linux/nmi.h
> +++ b/include/linux/nmi.h
> @@ -210,6 +210,7 @@ static inline bool trigger_single_cpu_backtrace(int cpu)
>  
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR_PERF
>  u64 hw_nmi_get_sample_period(int watchdog_thresh);
> +bool arch_perf_nmi_is_available(void);
>  #endif
>  
>  #if defined(CONFIG_HARDLOCKUP_CHECK_TIMESTAMP) && \
> diff --git a/kernel/watchdog_perf.c b/kernel/watchdog_perf.c
> index 349fcd4d2abc..8ea00c4a24b2 100644
> --- 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;
> +
> +	ret = hardlockup_detector_event_create();
>  
>  	if (ret) {
>  		pr_info("Perf NMI watchdog permanently disabled\n");
> -- 
> 2.40.1.698.g37aff9b760-goog
>