linux-kernel - Re: [PATCH RFC] xen-time: decreasing the rating of the xen clocksource below that of the tsc clocksource for dom0's

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Message-ID: <54AD65D4.9070101@oracle.com>
Date:	Wed, 07 Jan 2015 11:59:00 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Imre Palik <imrep.amz@...il.com>, xen-devel@...ts.xenproject.org
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, "Palik, Imre" <imrep@...zon.de>,
	Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCH RFC] xen-time: decreasing the rating of the xen clocksource
 below that of the tsc clocksource for dom0's

On 01/07/2015 11:16 AM, Imre Palik wrote:
> From: "Palik, Imre" <imrep@...zon.de>
>
> In Dom0's the use of the TSC clocksource (whenever it is stable enough to
> be used) instead of the Xen clocksource should not cause any issues, as
> Dom0 VMs never live-migrated.  The TSC clocksource is somewhat more
> efficient than the Xen paravirtualised clocksource, thus it should have
> higher rating.
>
> Cc: Anthony Liguori <aliguori@...zon.com>
> Signed-off-by: Imre Palik <imrep@...zon.de>
> ---
>   arch/x86/xen/time.c |    4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index f473d26..c768726 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -487,6 +487,10 @@ static void __init xen_time_init(void)
>   	int cpu = smp_processor_id();
>   	struct timespec tp;
>   
> +	/* As Dom0 is never moved, no penalty on using TSC there */
> +	if (xen_initial_domain())
> +		xen_clocksource.rating = 275;


Should this only be limited to dom0? Can we do this for guests running 
with TSC_MODE_NEVER_EMULATE as well?

We also have disable_migrate flag for guests (but it doesn't appear to 
be accessible to a guest kernel).

-boris


> +
>   	clocksource_register_hz(&xen_clocksource, NSEC_PER_SEC);
>   
>   	if (HYPERVISOR_vcpu_op(VCPUOP_stop_periodic_timer, cpu, NULL) == 0) {

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