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

Thomas Gleixner
2015-05-20T13:20:10Z

On Wed, 20 May 2015, Ingo Molnar wrote:
> * Borislav Petkov <bp@suse.de> wrote:
>
> > On Wed, May 20, 2015 at 01:11:20PM +0200, Ingo Molnar wrote:
> > > - MWAITX takes a 'timeout' parameter, but otherwise behaves exactly
> > > like MWAIT: i.e. once idle it won't exit idle on its own
> >
> > Let me quote the commit message:
> >
> > "MWAITT, another name is MWAITX (MWAIT with extensions), has a
> > configurable timer that causes MWAITX to exit on expiration."
>
> Ah. A useful skill that is, being able to read.
>
> > You need to set the second bit in ECX to enable the timer.
> >
> > I guess if you don't, then you get normal MWAIT but then you don't
> > need the timeout either...
>
> Yeah.
>
> So if it's a true timeout then we could use it to implement irq-less
> timers: that's actually pretty useful, because it could be faster than
> getting a local APIC timer irq, etc.

Uurgh, NO NO NO!

We have enough trouble with non functional timers already, we do not
need another variant of those.

We can supply the estimated sleep time though if that helps the PM
controller underneath to select a state. That's more or less what we
do in the governors as well.

Thanks,

tglx