Re: [PATCH 0/3] Improve TSC as a clocksource under VMware

[Jeff Hansen]

Alok,

No objections.  Looks good to me.

-Jeff

Alok Kataria wrote:
> Jeff, i have kept your Signed-off-by with this slightly modified patch,
> incase you have any objections to that please let me know.
>
> --
> [X86] Skip verification by the watchdog for TSC clocksource.
>
> From: Alok N Kataria <akataria@xxxxxxxxxx>
>
> This is achieved by resetting the CLOCKSOURCE_MUST_VERIFY flag.
>
> We add a tsc=reliable commandline option to enable this.
> This enables legacy hardware without HPET, LAPIC, or ACPI timers
> to enter high-resolution timer mode.
>
> Along with that have extended this to be used in virtualization
> environment, just for VMware as yet. This is important since there
> is a wrap-around problem with the acpi_pm timer.
> The acpi_pm counter is just 24bits and this can overflow in 4 seconds.
> With the NO_HZ kernels in virtualized environment, there can be situations
> when the guest is descheduled for longer duration, as a result we may miss
> the wrap of the acpi counter. When TSC is used as a clocksource and acpi_pm
> timer is being used as the watchdog clocksource this error in acpi_pm
> results in TSC being marked as unstable, and essentially results in time
> dropping in chunks of 4 seconds whenever this wrap is missed. Since the
> virtualized TSC is reliable on VMware, we should always use the TSCs
> clocksource on VMware, so we skip the verfication at runtime.
>
> Since we reset the flag for mgeode systems too, i have combined
> the mgeode case with the check for VMware.
>
> Signed-off-by: Jeff Hansen <jhansen@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Alok N Kataria <akataria@xxxxxxxxxx>
> Cc: Chris Snook <csnook@xxxxxxxxxx>
> ---
>
>  Documentation/kernel-parameters.txt |    7 +++++++
>  arch/x86/kernel/tsc.c               |   33 +++++++++++++++++++++------------
>  2 files changed, 28 insertions(+), 12 deletions(-)
>
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index e82c5bc..c5eb028 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2230,6 +2230,13 @@ and is between 256 and 4096 characters. It is defined in the file
>  			Format:
>  			<io>,<irq>,<dma>,<dma2>,<sb_io>,<sb_irq>,<sb_dma>,<mpu_io>,<mpu_irq>
>  
> +	tsc=		Disable clocksource-must-verify flag for TSC.
> +			Format: <string>
> +			[x86] reliable: mark tsc clocksource as reliable, this
> +			disables clocksource verification at runtime.
> +			Used to enable high-resolution timer mode on older
> +			hardware, and in virtualized environment.
> +
>  	turbografx.map[2|3]=	[HW,JOY]
>  			TurboGraFX parallel port interface
>  			Format:
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 4ae207c..24eff09 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -32,6 +32,7 @@ static int tsc_unstable;
>     erroneous rdtsc usage on !cpu_has_tsc processors */
>  static int tsc_disabled = -1;
>  
> +static int tsc_clocksource_reliable;
>  /*
>   * Scheduler clock - returns current time in nanosec units.
>   */
> @@ -99,6 +100,15 @@ int __init notsc_setup(char *str)
>  
>  __setup("notsc", notsc_setup);
>  
> +static int __init tsc_setup(char *str)
> +{
> +	if (!strcmp(str, "reliable"))
> +		tsc_clocksource_reliable = 1;
> +	return 1;
> +}
> +
> +__setup("tsc=", tsc_setup);
> +
>  #define MAX_RETRIES     5
>  #define SMI_TRESHOLD    50000
>  
> @@ -745,24 +755,21 @@ static struct dmi_system_id __initdata bad_tsc_dmi_table[] = {
>  	{}
>  };
>  
> -/*
> - * Geode_LX - the OLPC CPU has a possibly a very reliable TSC
> - */
> +static void __init check_system_tsc_reliable(void)
> +{
>  #ifdef CONFIG_MGEODE_LX
> -/* RTSC counts during suspend */
> +	/* RTSC counts during suspend */
>  #define RTSC_SUSP 0x100
> -
> -static void __init check_geode_tsc_reliable(void)
> -{
>  	unsigned long res_low, res_high;
>  
>  	rdmsr_safe(MSR_GEODE_BUSCONT_CONF0, &res_low, &res_high);
> +	/* Geode_LX - the OLPC CPU has a possibly a very reliable TSC */
>  	if (res_low & RTSC_SUSP)
> -		clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
> -}
> -#else
> -static inline void check_geode_tsc_reliable(void) { }
> +		tsc_clocksource_reliable = 1;
>  #endif
> +	if (boot_cpu_data.x86_hyper_vendor == X86_HYPER_VENDOR_VMWARE)
> +		tsc_clocksource_reliable = 1;
> +}
>  
>  /*
>   * Make an educated guess if the TSC is trustworthy and synchronized
> @@ -797,6 +804,8 @@ static void __init init_tsc_clocksource(void)
>  {
>  	clocksource_tsc.mult = clocksource_khz2mult(tsc_khz,
>  			clocksource_tsc.shift);
> +	if (tsc_clocksource_reliable)
> +		clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
>  	/* lower the rating if we already know its unstable: */
>  	if (check_tsc_unstable()) {
>  		clocksource_tsc.rating = 0;
> @@ -857,7 +866,7 @@ void __init tsc_init(void)
>  	if (unsynchronized_tsc())
>  		mark_tsc_unstable("TSCs unsynchronized");
>  
> -	check_geode_tsc_reliable();
> +	check_system_tsc_reliable();
>  	init_tsc_clocksource();
>  }
>  
>
>
>   


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