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

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Date:	Tue, 13 Jan 2015 11:33:30 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Imre Palik <imrep.amz@...il.com>,
	xen-devel@...ts.xenproject.org
CC:	"Palik, Imre" <imrep@...zon.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Anthony Liguori <aliguori@...zon.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH] xen-time: decreasing the rating of the xen
 clocksource below that of the tsc clocksource for dom0's

On 01/13/2015 11:17 AM, Boris Ostrovsky wrote:
> On 01/13/2015 11:07 AM, David Vrabel wrote:
>> On 13/01/15 15:42, Boris Ostrovsky wrote:
>>> On 01/13/2015 04:52 AM, David Vrabel wrote:
>>>> On 13/01/15 08:14, 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.
>>>>>
>>>>> This patch decreases the rating of the Xen clocksource in Dom0s to 
>>>>> 275.
>>>>> Which is half-way between the rating of the TSC clocksource (300) and
>>>>> the
>>>>> hpet clocksource (250).
>>>> I'm happy with this but would like to see acks from those who objected
>>>> to v1.
>>>>
>>>> David
>>>>
>>>>> --- 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 */
>>> Again, why not any PV guest with TSC_MODE_NEVER_EMULATE?
>> Surely if TSC_MODE_NEVER_EMULATE is set then the TSC is /not/ stable
>> across a guest save/restore thus the PV clocksource must be used?
>
> TSC is declared stable when !d->disable_migrate && !d->arch.vtsc, with 
> vtsc being 0 with TSC_MODE_NEVER_EMULATE (per domain_cpuid()).

I think I inverted domain_cpuid's logic here but my point was that we 
use'd use combination of TSC_MODE_NEVER_EMULATE and CPUID flag 
indicating TSC stability to pick the clocksource.


-boris

>
> And if TSC is not stable as seen by CPUID (which would be the case if 
> disable_migrate is not set) then kernel won't use TSC as clocksource 
> anyway, regardless of rating value, won't it?
>
> -boris
>
>>
>> I don't think we want to assume that TSC_MODE_NEVER_EMULATE => never
>> migrate.
>>
>> David
>

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