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

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Date:	Wed, 20 May 2015 16:51:22 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Borislav Petkov <bp@...e.de>, Huang Rui <ray.huang@....com>,
	Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Fengguang Wu <fengguang.wu@...el.com>,
	Aaron Lu <aaron.lu@...el.com>, Tony Li <tony.li@....com>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a
 configurable timer


* Thomas Gleixner <tglx@...utronix.de> wrote:

> On Wed, 20 May 2015, Ingo Molnar wrote:
> > * Borislav Petkov <bp@...e.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!

I know, I know :-)

The XP PIC was a nasty, broken hardware timer, and all x86 timer 
generations after that made the situation even worse.

> 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.

That's not what appears to be happening here though: the MWAITX will 
return after the timeout.

Which isn't really useful unless we use it to drive timers.

So 'lets not use it' might be the sane answer.

Thanks,

	Ingo
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