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Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option
To:     thibodux@gmail.com, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org
Cc:     oleksandr_andrushchenko@epam.com, tglx@linutronix.de,
        jgross@suse.com, ryan.thibodeaux@starlab.io
References: <1553279397-130201-1-git-send-email-ryan.thibodeaux@starlab.io>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <abf65a68-9463-0f0e-1f51-6eefd3cacc88@oracle.com>
Date:   Fri, 22 Mar 2019 18:10:16 -0400
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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);