Date:   Mon, 6 Feb 2023 17:08:32 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        john.stultz@linaro.org, sboyd@kernel.org, corbet@lwn.net,
        Mark.Rutland@arm.com, maz@kernel.org, kernel-team@meta.com,
        neeraju@codeaurora.org, ak@linux.intel.com, feng.tang@intel.com,
        zhengjun.xing@intel.com
Subject: Re: PATCH v2 clocksource 8/7] clocksource: Enable TSC watchdog
 checking of HPET and PMTMR only when requested
Message-ID: <20230207010832.GU2948950@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20230125002708.GA1471122@paulmck-ThinkPad-P17-Gen-1>
 <20230203043658.GA1513624@paulmck-ThinkPad-P17-Gen-1>
 <82e4dae9-0aca-a06b-cb0d-5a189998cdc1@redhat.com>
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On Mon, Feb 06, 2023 at 02:57:55PM -0500, Waiman Long wrote:
> 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>

Applied, thank you!

							Thanx, Paul