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Re: [Xen-devel] [PATCH v13] tolerate jitter in cpu_khz calculation to avoid TSC emulation
- To: Olaf Hering <olaf@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Thu, 14 Mar 2019 15:10:16 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 14 Mar 2019 14:10:23 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 14/03/2019 14:50, Olaf Hering wrote:
> Am Wed, 13 Mar 2019 03:18:39 -0600
> schrieb "Jan Beulich" <JBeulich@xxxxxxxx>:
>
>> I'm sorry, but I continue to object to this adjustment getting done
>> both by default _and_ not in a per-guest manner. As said before,
>> you can't demand guests to run NTP, and hence you can't expect
>> them to get along with a few hundred kHz jump in observed TSC
>> frequency. Whether the performance drop due to vTSC use is
>> better or worse is a policy decision, which we should leave to the
>> admin. Hence the feature needs to be off by default, and there
>> needs to be at least a host-wide control to enable it; a per-guest
>> control would be better. IOW I explicitly do not agree with the
>> last sentence of the commit message.
>
> So this seems the be the essential part that prevents moving forward.
>
> Your claim is basically that "we do not know how the workload reacts
> to frequency change".
> My claim is basically "there is enough evidence that syncing with
> external clock is required if the frequency remotely matters".
>
> I think that conflict can not be easily solved.
>
> One way to solve it would be a knob that injects a value into the
> proposed "vtsc_tolerance_khz" variable, leave the calculation to
> the host admin, and leave code in tsc_set_info basically as is.
>
> Maybe "xl set-params" can be the way to change the value, that way
> it can be changed globally at runtime if needed.
>
> In staging the change would affect HVM and PVH. I never ran PVH,
> I have to assume it behaves like HVM in this regard.
I think Andrew (or Ian?) once suggested to handle this whole mess in the
migration stream instead of the hypervisor.
So why don't you:
- add a domain config item for specifying the allowed jitter
- add that value to the migration struct xc_sr_rec_tsc_info (there is a
reserved field available)
- and then modify handle_tsc_info() in tools/libxc/xc_sr_common_x86.c to
test the host frequency to be in the acceptable range and if this is
the case put the host frequency into the gtsc_khz parameter of the
xc_domain_set_tsc_info() call
This would be the least intrusive change allowing maximum flexibility
IMO.
Juergen
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