Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for qualified platforms

[Feng Tang]

On Wed, Nov 17, 2021 at 10:37:51AM +0800, Feng Tang wrote:
> There are cases that tsc clocksources are wrongly judged as unstable by
> clocksource watchdogs like hpet, acpi_pm or 'refined-jiffies'. While
> there is hardly a general reliable way to check the validity of a
> watchdog, and to protect the innocent tsc, Thomas Gleixner proposed [1]:

Hi All,

Some more update, last week we got report from validation team that
the "tsc judged as unstable" happened on latest desktop platform,
which has serial earlyprintk enabled, and the watchdog here is
'refined-jiffies' while hpet is disabled during the PC10 check. I
tried severy other client platforms I can find: Kabylake, Icelake
and Alderlake, and the mis-judging can be easily reproduced on
Icelake and Alderlake (not on Kabylake). Which could be cued by
this 2/2 patch.

Also, today we got same report on a 2-sockets Icelake Server with
5.5 kernel, while the watchdog is 'hpet', and the system is running
stressful big-data workload.

Thanks,
Feng


> "I'm inclined to lift that requirement when the CPU has:
> 
>     1) X86_FEATURE_CONSTANT_TSC
>     2) X86_FEATURE_NONSTOP_TSC
>     3) X86_FEATURE_NONSTOP_TSC_S3
>     4) X86_FEATURE_TSC_ADJUST
>     5) At max. 4 sockets
> 
>  After two decades of horrors we're finally at a point where TSC seems
>  to be halfway reliable and less abused by BIOS tinkerers. TSC_ADJUST
>  was really key as we can now detect even small modifications reliably
>  and the important point is that we can cure them as well (not pretty
>  but better than all other options)."
> 
> As feature #3 X86_FEATURE_NONSTOP_TSC_S3 only exists on several generations
> of Atom processor, and is always coupled with X86_FEATURE_CONSTANT_TSC
> and X86_FEATURE_NONSTOP_TSC, skip checking it, and also be more defensive
> to use maxim of 2 sockets.
> 
> The check is done inside tsc_init() before registering 'tsc-early' and
> 'tsc' clocksources, as there were cases that both of them had been
> wrongly judged as unreliable.
> 
> For more background of tsc/watchdog, there is a good summary in [2]
> 
> [1]. https://lore.kernel.org/lkml/87eekfk8bd.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> [2]. https://lore.kernel.org/lkml/87a6pimt1f.ffs@xxxxxxxxxxxxxxxxxxxxxxx/
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> ---
> Change log:
> 
>   v3:
>     * rebased against 5.16-rc1
>     * refine commit log
> 
>   v2:
>     * Directly skip watchdog check without messing flag
>       'tsc_clocksource_reliable' (Thomas)
> 
>  arch/x86/kernel/tsc.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 2e076a459a0c..389511f59101 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -1180,6 +1180,12 @@ void mark_tsc_unstable(char *reason)
>  
>  EXPORT_SYMBOL_GPL(mark_tsc_unstable);
>  
> +static void __init tsc_skip_watchdog_verify(void)
> +{
> +	clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
> +	clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
> +}
> +
>  static void __init check_system_tsc_reliable(void)
>  {
>  #if defined(CONFIG_MGEODEGX1) || defined(CONFIG_MGEODE_LX) || defined(CONFIG_X86_GENERIC)
> @@ -1196,6 +1202,17 @@ static void __init check_system_tsc_reliable(void)
>  #endif
>  	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
>  		tsc_clocksource_reliable = 1;
> +
> +	/*
> +	 * Ideally the socket number should be checked, but this is called
> +	 * by tsc_init() which is in early boot phase and the socket numbers
> +	 * may not be available. Use 'nr_online_nodes' as a fallback solution
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> +	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
> +	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
> +	    nr_online_nodes <= 2)
> +		tsc_skip_watchdog_verify();
>  }
>  
>  /*
> @@ -1387,9 +1404,6 @@ static int __init init_tsc_clocksource(void)
>  	if (tsc_unstable)
>  		goto unreg;
>  
> -	if (tsc_clocksource_reliable || no_tsc_watchdog)
> -		clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
> -
>  	if (boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))
>  		clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
>  
> @@ -1527,7 +1541,7 @@ void __init tsc_init(void)
>  	}
>  
>  	if (tsc_clocksource_reliable || no_tsc_watchdog)
> -		clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
> +		tsc_skip_watchdog_verify();
>  
>  	clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
>  	detect_art();
> -- 
> 2.27.0