Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a configurable timer

[Huang Rui]

On Wed, May 20, 2015 at 05:12:13PM +0800, Borislav Petkov wrote:
> On Wed, May 20, 2015 at 10:55:20AM +0200, Ingo Molnar wrote:
> > Does it use it to decide how 'deep' a sleep it will go into, i.e. 
> > larger timeouts cause longer entry and exit latencies?
> 
> That's what the HLT thing does. Cores go into C1 and then at some point
> (hysteresis, etc) the whole core complex enters C1E.
> 
> The MWAIT* should be used for only shorter sleeps as it remains in C1.
> IMHO, of course.
> 
> But the problem there is another: what happens if the timeout fires,
> you wake up and see that you can remain idle? Do HLT? Do another MWAITX
> round?
> 
> This means you have an additional unnecessary wakeup which costs.
> 
> > I suppose it's also the case that if an interrupt arrives _before_ the 
> > expected timeout then MWAITX will try to exit immediately, it won't 
> > wait until the timeout, right?
> 
> I'd assume so - I mean, it must, right.
> 
> BUT!, in talking to Andy about it last night on IRC, he pointed out
> that when using acpi_idle, we never come to calling x86_idle() and from
> looking quickly at cpuidle_idle_call(), that still might be the case as
> we go to use_default only when there's an error with the cpuidle driver
> or so.
> 
> So Rui, before you go and do more work on it, you should probably
> analyze what cpuidle exactly does (if you haven't done so yet). And on
> AMD we do use acpi_idle - at least on my F15h box that is the case:
> 
> $ grep . /sys/devices/system/cpu/cpuidle/current_*
> /sys/devices/system/cpu/cpuidle/current_driver:acpi_idle
> /sys/devices/system/cpu/cpuidle/current_governor_ro:menu
> 

OK, I know. Thanks to reminder. :)

Thanks,
Rui
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