Re: [Xen-devel] [PATCH] x86/xen: Add "xen_timer_slop" command line option

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 3/22/19 2:29 PM, thibodux@xxxxxxxxx wrote:
> From: Ryan Thibodeaux <ryan.thibodeaux@xxxxxxxxxx>
>
> Add a new command-line option "xen_timer_slop=<INT>" that sets the
> minimum delta of virtual Xen timers. This commit does not change the
> default timer slop value for virtual Xen timers.
>
> Lowering the timer slop value should improve the accuracy of virtual
> timers (e.g., better process dispatch latency), but it will likely
> increase the number of virtual timer interrupts (relative to the
> original slop setting).
>
> The original timer slop value has not changed since the introduction
> of the Xen-aware Linux kernel code. This commit provides users an
> opportunity to tune timer performance given the refinements to
> hardware and the Xen event channel processing. It also mirrors
> a feature in the Xen hypervisor - the "timer_slop" Xen command line
> option.

Is there any data that shows effects of using this new parameter?

-boris


>
> Signed-off-by: Ryan Thibodeaux <ryan.thibodeaux@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  7 +++++++
>  arch/x86/xen/time.c                             | 18 ++++++++++++++++--
>  2 files changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt 
> b/Documentation/admin-guide/kernel-parameters.txt
> index 858b6c0..fb58c84 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5129,6 +5129,13 @@
>                       with 
> /sys/devices/system/xen_memory/xen_memory0/scrub_pages.
>                       Default value controlled with 
> CONFIG_XEN_SCRUB_PAGES_DEFAULT.
>  
> +     xen_timer_slop= [X86-64,XEN]
> +                     Set the timer slop (in nanoseconds) for the virtual Xen
> +                     timers (default is 100000). This adjusts the minimum
> +                     delta of virtualized Xen timers, where lower values
> +                     improve timer resolution at the expense of processing
> +                     more timer interrupts.
> +
>       xirc2ps_cs=     [NET,PCMCIA]
>                       Format:
>                       
> <irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 6e29794..0393968 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -212,7 +212,7 @@ static int xen_timerop_set_next_event(unsigned long delta,
>       return 0;
>  }
>  
> -static const struct clock_event_device xen_timerop_clockevent = {
> +static struct clock_event_device xen_timerop_clockevent __ro_after_init = {
>       .name                   = "xen",
>       .features               = CLOCK_EVT_FEAT_ONESHOT,
>  
> @@ -273,7 +273,7 @@ static int xen_vcpuop_set_next_event(unsigned long delta,
>       return ret;
>  }
>  
> -static const struct clock_event_device xen_vcpuop_clockevent = {
> +static struct clock_event_device xen_vcpuop_clockevent __ro_after_init = {
>       .name = "xen",
>       .features = CLOCK_EVT_FEAT_ONESHOT,
>  
> @@ -570,3 +570,17 @@ void __init xen_hvm_init_time_ops(void)
>       x86_platform.set_wallclock = xen_set_wallclock;
>  }
>  #endif
> +
> +/* Kernel parameter to specify Xen timer slop */
> +static int __init parse_xen_timer_slop(char *ptr)
> +{
> +     unsigned long slop = memparse(ptr, NULL);
> +
> +     xen_timerop_clockevent.min_delta_ns = slop;
> +     xen_timerop_clockevent.min_delta_ticks = slop;
> +     xen_vcpuop_clockevent.min_delta_ns = slop;
> +     xen_vcpuop_clockevent.min_delta_ticks = slop;
> +
> +     return 0;
> +}
> +early_param("xen_timer_slop", parse_xen_timer_slop);


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