Re: RFC: disable HPET legacy mode after timer check

[Simon Gaiser <simon@invisiblethingslab.com>]

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Andrew Cooper:
[...]
>> It reaches low power states only for a fraction of the suspend to idle
>> time, so something still makes the CPU/chipset think it should leave the
>> low power mode, but that's another topic.
> 
> Do you have any further info here?  There are a range of possibilities,
> from excess timers in Xen (e.g. PV guests default to a 100Hz timer even
> though no guests actually want it AFAICT), or the 1s TSC rendezvous
> (which isn't actually needed on modern systems), all the way to the
> platform devices not entering d3hot.

So in the meantime I got some progress here.

What helps a lot is setting cpufreq to powersave before going to s2idle.
With that I get residency of about 88 % (everything is still tested with
only dom0 running). Not yet the > 99 % that a native Linux manages, but
much better than before (<< 50 %).

While, based on your and Marek's feedback, I was already looking at
active timers, I first ignored the cpufreq dbs timer since the idle
driver suspend it and I assumed it was active because I wake things up
when triggering the debug key. But turns out disabling the ondemand
governor has a big effect. But not sure if it's the timer itself or some
other part of it.

I tried to disable the time calibration timer. While eyeballing on the
power meter I first thought it brings some improvement there's no
difference according to the residency counters (will need to improve my
power measurement setup).

Other timers I see active:

common/sched/core.c#vcpu_singleshot_timer_fn:
If I understand correctly those are configure by the domain (so dom0
here). So Linux should do this right. But I will have to have a closer
look.

arch/x86/cpu/mcheck/non-fatal.c#mce_work_fn:
Triggers only seldom (something >> 1 s), so unlikely. But will try
disabling.

arch/x86/time.c#plt_overflow:
Dito.

Simon

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