linux-kernel - Re: [PATCH v3] x86/tsc: Disable clocksource watchdog for TSC on recent UV

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Message-ID: <20264c7b-36dd-4fd3-a755-3f46584f37bc@intel.com>
Date: Tue, 7 Oct 2025 09:48:58 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Dimitri Sivanich <sivanich@....com>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc: Jiri Wiesner <jwiesner@...e.de>, Steve Wahl <steve.wahl@....com>,
 Justin Ernst <justin.ernst@....com>, Kyle Meyer <kyle.meyer@....com>,
 Russ Anderson <russ.anderson@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 Marco Elver <elver@...gle.com>, "Guilherme G. Piccoli"
 <gpiccoli@...lia.com>, Nikunj A Dadhania <nikunj@....com>,
 "Xin Li (Intel)" <xin@...or.com>, Dimitri Sivanich <dimitri.sivanich@....com>
Subject: Re: [PATCH v3] x86/tsc: Disable clocksource watchdog for TSC on
 recent UV

On 10/7/25 09:32, Dimitri Sivanich wrote:
> +static inline int is_uvx_hub(void) { return 0; }
> +static inline int is_uvy_hub(void) { return 0; }
> +static inline int is_uv_hub(void) { return 0; }
>  
>  #endif	/* X86_UV */
>  
> diff --git a/arch/x86/include/asm/uv/uv_hub.h b/arch/x86/include/asm/uv/uv_hub.h
> index ea877fd83114..6e085ce8fc02 100644
> --- a/arch/x86/include/asm/uv/uv_hub.h
> +++ b/arch/x86/include/asm/uv/uv_hub.h
> @@ -246,6 +246,7 @@ static inline int is_uv5_hub(void) { return is_uv(UV5); }
>   * then test if is UV4.
>   */
>  
> +#ifdef CONFIG_X86_UV
>  /* UVX class: UV2,3,4 */
>  static inline int is_uvx_hub(void) { return is_uv(UVX); }
>  
> @@ -254,6 +255,7 @@ static inline int is_uvy_hub(void) { return is_uv(UVY); }
>  
>  /* Any UV Hubbed System */
>  static inline int is_uv_hub(void) { return is_uv(UV_ANY); }
> +#endif

Defining those helpers across two different headers seems like a recipe
for pain.

I suspect a big chunk of those stubs (and their #ifdefs could completely
go away if you _just_ did:

#ifdef CONFIG_X86_UV
static inline int uv_hub_type(void)
{
        return uv_hub_info->hub_type;
}
#else
static inline int uv_hub_type(void)
{
	return 0;
}
#endif

In any case, this is precisely the kind of patch that would be best
refactored into two piece: one to expose the is_uv...() function and
another to actually use it.

Also, at this point, this:

>  	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
>  	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
>  	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
> -	    topology_max_packages() <= 4)
> +	    (topology_max_packages() <= 4 || is_uvy_hub()))
>  		tsc_disable_clocksource_watchdog();

has IMNHO gotten out of hand.

It should probably be:

 	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
 	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
 	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
	    platform_is_exempt_from_watchdog())
 		tsc_disable_clocksource_watchdog();

In addition, 233756a640be talked quite a bit about *why* the 4-socket
line was chosen. This needs to have a similar explanation for UV systems.