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

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

[Dimitri Sivanich]

Disable clocksource watchdog checking on recent and future UV platforms
to avoid false positives.

Commits 'b50db7095fe0 ("x86/tsc: Disable clocksource watchdog for TSC on
qualified platforms")' and '233756a640be ("Extend watchdog check exemption
to 4-Sockets platform")' were introduced to avoid an issue where the TSC
is falsely declared unstable by exempting qualified platforms of up to
4-sockets from TSC clocksource watchdog checking.  Extend that exemption
to include recent and future UV platforms.

Signed-off-by: Dimitri Sivanich <sivanich@hpe.com>
---

Version 3:  Fixes a build error in the X86 32-bit build.

Version 2:  Fixes a build error that occurs when UV is not configured.

 arch/x86/include/asm/uv/uv.h     | 3 +++
 arch/x86/include/asm/uv/uv_hub.h | 2 ++
 arch/x86/kernel/tsc.c            | 3 ++-
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uv/uv.h b/arch/x86/include/asm/uv/uv.h
index 648eb23fe7f0..f1aebdf85e68 100644
--- a/arch/x86/include/asm/uv/uv.h
+++ b/arch/x86/include/asm/uv/uv.h
@@ -38,6 +38,9 @@ static inline int is_uv_system(void)	{ return 0; }
 static inline int is_uv_hubbed(int uv)	{ return 0; }
 static inline void uv_cpu_init(void)	{ }
 static inline void uv_system_init(void)	{ }
+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
 
 union uvh_apicid {
     unsigned long       v;
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 87e749106dda..a9f97d423ab0 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -32,6 +32,7 @@
 #include <asm/msr.h>
 #include <asm/topology.h>
 #include <asm/uv/uv.h>
+#include <asm/uv/uv_hub.h>
 #include <asm/sev.h>
 
 unsigned int __read_mostly cpu_khz;	/* TSC clocks / usec, not used here */
@@ -1261,7 +1262,7 @@ static void __init check_system_tsc_reliable(void)
 	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();
 }
 
-- 
2.43.0

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

[Dimitri Sivanich]

On Wed, Aug 20, 2025 at 02:54:16PM -0500, Dimitri Sivanich wrote:
> Disable clocksource watchdog checking on recent and future UV platforms
> to avoid false positives.
> 
> Commits 'b50db7095fe0 ("x86/tsc: Disable clocksource watchdog for TSC on
> qualified platforms")' and '233756a640be ("Extend watchdog check exemption
> to 4-Sockets platform")' were introduced to avoid an issue where the TSC
> is falsely declared unstable by exempting qualified platforms of up to
> 4-sockets from TSC clocksource watchdog checking.  Extend that exemption
> to include recent and future UV platforms.
> 
> Signed-off-by: Dimitri Sivanich <sivanich@hpe.com>
> ---
> 
> Version 3:  Fixes a build error in the X86 32-bit build.
> 
> Version 2:  Fixes a build error that occurs when UV is not configured.
> 
>  arch/x86/include/asm/uv/uv.h     | 3 +++
>  arch/x86/include/asm/uv/uv_hub.h | 2 ++
>  arch/x86/kernel/tsc.c            | 3 ++-
>  3 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/uv/uv.h b/arch/x86/include/asm/uv/uv.h
> index 648eb23fe7f0..f1aebdf85e68 100644
> --- a/arch/x86/include/asm/uv/uv.h
> +++ b/arch/x86/include/asm/uv/uv.h
> @@ -38,6 +38,9 @@ static inline int is_uv_system(void)	{ return 0; }
>  static inline int is_uv_hubbed(int uv)	{ return 0; }
>  static inline void uv_cpu_init(void)	{ }
>  static inline void uv_system_init(void)	{ }
> +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
>  
>  union uvh_apicid {
>      unsigned long       v;
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 87e749106dda..a9f97d423ab0 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -32,6 +32,7 @@
>  #include <asm/msr.h>
>  #include <asm/topology.h>
>  #include <asm/uv/uv.h>
> +#include <asm/uv/uv_hub.h>
>  #include <asm/sev.h>
>  
>  unsigned int __read_mostly cpu_khz;	/* TSC clocks / usec, not used here */
> @@ -1261,7 +1262,7 @@ static void __init check_system_tsc_reliable(void)
>  	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();
>  }
>  
> -- 
> 2.43.0

Just a friendly reminder about this patch.

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

[Dimitri Sivanich]

Disable clocksource watchdog checking on recent and future UV platforms
to avoid false positives.

Commits 'b50db7095fe0 ("x86/tsc: Disable clocksource watchdog for TSC on
qualified platforms")' and '233756a640be ("Extend watchdog check exemption
to 4-Sockets platform")' were introduced to avoid an issue where the TSC
is falsely declared unstable by exempting qualified platforms of up to
4-sockets from TSC clocksource watchdog checking.  Extend that exemption
to include recent and future UV platforms.

Signed-off-by: Dimitri Sivanich <sivanich@hpe.com>
---

Version 3:  Fixes a build error in the X86 32-bit build.

Version 2:  Fixes a build error that occurs when UV is not configured.

 arch/x86/include/asm/uv/uv.h     | 3 +++
 arch/x86/include/asm/uv/uv_hub.h | 2 ++
 arch/x86/kernel/tsc.c            | 3 ++-
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uv/uv.h b/arch/x86/include/asm/uv/uv.h
index 648eb23fe7f0..f1aebdf85e68 100644
--- a/arch/x86/include/asm/uv/uv.h
+++ b/arch/x86/include/asm/uv/uv.h
@@ -38,6 +38,9 @@ static inline int is_uv_system(void)	{ return 0; }
 static inline int is_uv_hubbed(int uv)	{ return 0; }
 static inline void uv_cpu_init(void)	{ }
 static inline void uv_system_init(void)	{ }
+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
 
 union uvh_apicid {
     unsigned long       v;
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 87e749106dda..a9f97d423ab0 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -32,6 +32,7 @@
 #include <asm/msr.h>
 #include <asm/topology.h>
 #include <asm/uv/uv.h>
+#include <asm/uv/uv_hub.h>
 #include <asm/sev.h>
 
 unsigned int __read_mostly cpu_khz;	/* TSC clocks / usec, not used here */
@@ -1261,7 +1262,7 @@ static void __init check_system_tsc_reliable(void)
 	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();
 }
 
-- 
2.43.0

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

[Dave Hansen]

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.

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

[Dimitri Sivanich]

On Tue, Oct 07, 2025 at 09:48:58AM -0700, Dave Hansen wrote:
> 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.

I will add the following to the description:

"HPE UV hardware and firmware is designed to ensure a reliable and synchronized
 TSC mechanism.  Comparing the TSC against secondary clocksources can result in
 false positives due to variable access latency caused by system traffic.  The
 best course of action against these false positives has been found to simply
 disable watchdog checking of the TSC.  Currently we recommend that customers
 apply 'tsc=nowatchdog' to the kernel command line."

The requested code changes will be forthcoming.