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

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Message-ID: <20150520085520.GA8566@gmail.com>
Date:	Wed, 20 May 2015 10:55:20 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...e.de>
Cc:	Huang Rui <ray.huang@....com>, Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Gleixner <tglx@...utronix.de>, 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>
Subject: Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a
 configurable timer


* Borislav Petkov <bp@...e.de> wrote:

> On Tue, May 19, 2015 at 04:01:10PM +0800, Huang Rui wrote:
> > MWAITX/MWAIT does not let the cpu core go into C1 state on AMD processors.
> > The cpu core still consumes less power while waiting, and has faster exit
> > from waiting than "Halt". This patch implements an interface using the
> > kernel parameter "idle=" to configure mwaitx type and timer value.
> > 
> > If "idle=mwaitx", the timeout will be set as the maximum value
> > ((2^64 - 1) * TSC cycle).
> > If "idle=mwaitx,100", the timeout will be set as 100ns.
> > If the processor doesn't support MWAITX, then halt is used.

So what does the hardware do with the timeout value?

Does it use it to decide how 'deep' a sleep it will go into, i.e. 
larger timeouts cause longer entry and exit latencies?

Or some other purpose?

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?

Thanks,

	Ingo
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