Re: [PATCH v6 29/29] x86/tsc: Switch to perf-based hardlockup detector if TSC become unstable

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Ricardo Neri's message of May 6, 2022 10:00 am:
> The HPET-based hardlockup detector relies on the TSC to determine if an
> observed NMI interrupt was originated by HPET timer. Hence, this detector
> can no longer be used with an unstable TSC.
> 
> In such case, permanently stop the HPET-based hardlockup detector and
> start the perf-based detector.
> 
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Stephane Eranian <eranian@google.com>
> Cc: "Ravi V. Shankar" <ravi.v.shankar@intel.com>
> Cc: iommu@lists.linux-foundation.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: x86@kernel.org
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Changes since v5:
>  * Relocated the delcaration of hardlockup_detector_switch_to_perf() to
>    x86/nmi.h It does not depend on HPET.
>  * Removed function stub. The shim hardlockup detector is always for x86.
> 
> Changes since v4:
>  * Added a stub version of hardlockup_detector_switch_to_perf() for
>    !CONFIG_HPET_TIMER. (lkp)
>  * Reconfigure the whole lockup detector instead of unconditionally
>    starting the perf-based hardlockup detector.
> 
> Changes since v3:
>  * None
> 
> Changes since v2:
>  * Introduced this patch.
> 
> Changes since v1:
>  * N/A
> ---
>  arch/x86/include/asm/nmi.h     | 6 ++++++
>  arch/x86/kernel/tsc.c          | 2 ++
>  arch/x86/kernel/watchdog_hld.c | 6 ++++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/arch/x86/include/asm/nmi.h b/arch/x86/include/asm/nmi.h
> index 4a0d5b562c91..47752ff67d8b 100644
> --- a/arch/x86/include/asm/nmi.h
> +++ b/arch/x86/include/asm/nmi.h
> @@ -63,4 +63,10 @@ void stop_nmi(void);
>  void restart_nmi(void);
>  void local_touch_nmi(void);
>  
> +#ifdef CONFIG_X86_HARDLOCKUP_DETECTOR
> +void hardlockup_detector_switch_to_perf(void);
> +#else
> +static inline void hardlockup_detector_switch_to_perf(void) { }
> +#endif
> +
>  #endif /* _ASM_X86_NMI_H */
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index cc1843044d88..74772ffc79d1 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -1176,6 +1176,8 @@ void mark_tsc_unstable(char *reason)
>  
>  	clocksource_mark_unstable(&clocksource_tsc_early);
>  	clocksource_mark_unstable(&clocksource_tsc);
> +
> +	hardlockup_detector_switch_to_perf();
>  }
>  
>  EXPORT_SYMBOL_GPL(mark_tsc_unstable);
> diff --git a/arch/x86/kernel/watchdog_hld.c b/arch/x86/kernel/watchdog_hld.c
> index ef11f0af4ef5..7940977c6312 100644
> --- a/arch/x86/kernel/watchdog_hld.c
> +++ b/arch/x86/kernel/watchdog_hld.c
> @@ -83,3 +83,9 @@ void watchdog_nmi_start(void)
>  	if (detector_type == X86_HARDLOCKUP_DETECTOR_HPET)
>  		hardlockup_detector_hpet_start();
>  }
> +
> +void hardlockup_detector_switch_to_perf(void)
> +{
> +	detector_type = X86_HARDLOCKUP_DETECTOR_PERF;

Another possible problem along the same lines here,
isn't your watchdog still running at this point? And
it uses detector_type in the switch.

> +	lockup_detector_reconfigure();

Actually the detector_type switch is used in some
functions called by lockup_detector_reconfigure()
e.g., watchdog_nmi_stop, so this seems buggy even
without concurrent watchdog.

Is this switching a good idea in general? The admin
has asked for non-standard option because they want
more PMU counterss available and now it eats a
counter potentially causing a problem rather than
detecting one.

I would rather just disable with a warning if it were
up to me. If you *really* wanted to be fancy then
allow admin to re-enable via proc maybe.

Thanks,
Nick