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

Boris Ostrovsky
2019-03-22T22:11:00Z

On 3/22/19 2:29 PM, thibodux@gmail.com wrote:
> From: Ryan Thibodeaux <ryan.thibodeaux@starlab.io>
>
> 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@starlab.io>
> ---
> 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);