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

Waiman Long
2023-02-06T19:58:53Z

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>