Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option

Boris Ostrovsky
2019-03-25T13:44:07Z

On 3/25/19 8:05 AM, luca abeni wrote:
> Hi all,
>
> On Sat, 23 Mar 2019 11:41:51 +0100
> luca abeni <luca.abeni@santannapisa.it> wrote:
> [...]
>>>> Is there any data that shows effects of using this new parameter?
>>>>
>>> Yes, I've done some research and experiments on this. I did it
>>> together with a friend, which I'm Cc-ing, as I'm not sure we're
>>> ready/capable to share the results, yet (Luca?).
>> I think we can easily share the experimental data (cyclictest output
>> and plots).
>>
>> Moreover, we can share the scripts and tools for running the
>> experiments (so, everyone can easily reproduce the numbers by simply
>> typing "make" and waiting for some time :)
>>
>>
>> I'll try to package the results and the scripts/tools this evening,
>> and I'll send them.
> Sorry for the delay. I put some quick results here:
> http://retis.santannapisa.it/luca/XenTimers/
> (there also is a link to the scripts to be used for reproducing the
> results). The latencies have been measured by running cyclictest in the
> guest (see the scripts for details).
>
> The picture shows the latencies measured with an unpatched guest kernel
> and with a guest kernel having TIMER_SLOP set to 1000 (arbitrary small
> value :).
> All the experiments have been performed booting the hypervisor with a
> small timer_slop (the hypervisor's one) value. So, they show that
> decreasing the hypervisor's timer_slop is not enough to measure low
> latencies with cyclictest.

I have a couple of questions:
* Does it make sense to make this a tunable for other clockevent devices
as well?
* This patch adjusts min value. Could max value (ever) need a similar
adjustment?

-boris