Re: PATCH v2 clocksource 8/7] clocksource: Enable TSC watchdog checking of HPET and PMTMR only when requested

[Waiman Long <longman@redhat.com>]

On 2/2/23 23:36, Paul E. McKenney wrote:
> Unconditionally enabling TSC watchdog checking of the HPET and PMTMR
> clocksources can degrade latency and performance.  Therefore, provide
> a new "watchdog" option to the tsc= boot parameter that opts into such
> checking.  Note that tsc=watchdog is overridden by a tsc=nowatchdog
> regardless of their relative positions in the list of boot parameters.
>
> Reported-by: Thomas Gleixner <tglx@linutronix.de>
> Reported-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 95f0d104c2322..7b4df6d89d3c3 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6373,6 +6373,12 @@
>   			(HPET or PM timer) on systems whose TSC frequency was
>   			obtained from HW or FW using either an MSR or CPUID(0x15).
>   			Warn if the difference is more than 500 ppm.
> +			[x86] watchdog: Use TSC as the watchdog clocksource with
> +			which to check other HW timers (HPET or PM timer), but
> +			only on systems where TSC has been deemed trustworthy.
> +			This will be suppressed by an earlier tsc=nowatchdog and
> +			can be overridden by a later tsc=nowatchdog.  A console
> +			message will flag any such suppression or overriding.
>   
>   	tsc_early_khz=  [X86] Skip early TSC calibration and use the given
>   			value instead. Useful when the early TSC frequency discovery
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index a5371c6d4b64b..306c233c98d84 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -294,6 +294,7 @@ __setup("notsc", notsc_setup);
>   
>   static int no_sched_irq_time;
>   static int no_tsc_watchdog;
> +static int tsc_as_watchdog;
>   
>   static int __init tsc_setup(char *str)
>   {
> @@ -303,10 +304,22 @@ static int __init tsc_setup(char *str)
>   		no_sched_irq_time = 1;
>   	if (!strcmp(str, "unstable"))
>   		mark_tsc_unstable("boot parameter");
> -	if (!strcmp(str, "nowatchdog"))
> +	if (!strcmp(str, "nowatchdog")) {
>   		no_tsc_watchdog = 1;
> +		if (tsc_as_watchdog)
> +			pr_alert("%s: Overriding earlier tsc=watchdog with tsc=nowatchdog\n",
> +				 __func__);
> +		tsc_as_watchdog = 0;
> +	}
>   	if (!strcmp(str, "recalibrate"))
>   		tsc_force_recalibrate = 1;
> +	if (!strcmp(str, "watchdog")) {
> +		if (no_tsc_watchdog)
> +			pr_alert("%s: tsc=watchdog overridden by earlier tsc=nowatchdog\n",
> +				 __func__);
> +		else
> +			tsc_as_watchdog = 1;
> +	}
>   	return 1;
>   }
>   
> @@ -1192,7 +1205,8 @@ static void __init tsc_disable_clocksource_watchdog(void)
>   
>   bool tsc_clocksource_watchdog_disabled(void)
>   {
> -	return !(clocksource_tsc.flags & CLOCK_SOURCE_MUST_VERIFY);
> +	return !(clocksource_tsc.flags & CLOCK_SOURCE_MUST_VERIFY) &&
> +	       tsc_as_watchdog && !no_tsc_watchdog;
>   }
>   
>   static void __init check_system_tsc_reliable(void)
>
Acked-by: Waiman Long <longman@redhat.com>