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

[David Vrabel <david.vrabel@citrix.com>]

On 08/01/15 15:06, Imre Palik wrote:
> On 01/07/15 17:30, Ian Campbell wrote:
>> On Wed, 2015-01-07 at 17:16 +0100, Imre Palik wrote:
>>> From: "Palik, Imre" <imrep@amazon.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.
>>
>> Is this still true given that dom0's vcpus are migrated amongst pcpus on
>> the host? The tsc are not synchronised on some generations of hardware
>> so the result there would be the TSC appearing to do very odd things
>> under dom0's feet. Does Linux cope with that or does it not matter for
>> some other reason?
> 
> First of all, if the initial pcpus are not synchronised, linux won't consider
> TSC as a viable clocksource.
> 
> If the initial pcpus are synchronised, but then the dom0 vcpus are migrated to
> unsynchronised pcpus, then hopefully the tsc watchdog catches the issue, and
> the kernel falls back to the xen clocksource.  The issue here is that the
> watchdog can only detect clock skews bigger than 0.0625s/0.5s.  Hopefully this
> is enough to avoid the weirdest things.

I don't think any such hardware exists.  Either TSC is synchronized
across all CPUs or none.

> Also, some parts of the kernel (e.g., scheduling) will always use the paravirt
> clock.  No matter what priorities are set.
> 
> So it should be safe for some definition of safe.
> But I was unable to test it on a hardware platform that would hit the problematic
> case described above.

Ok.  Can you list the various time sources and their ratings in the
commit message for clarity.  i.e, to justify 275 (below TSC = 300. above
hpet = 250).

David