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

Petr Mladek
2023-05-26T12:36:58Z

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@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.
>
> 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