linux-kernel - Re: Mwait usage on AMD processors

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Message-ID: <20150514133857.GB6552@hr-slim.amd.com>
Date:	Thu, 14 May 2015 21:38:57 +0800
From:	Huang Rui <ray.huang@....com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	<linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	"Aaron Lu" <aaron.lu@...el.com>, Tony Li <tony.li@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Mwait usage on AMD processors

On Thu, May 14, 2015 at 01:23:03PM +0200, Borislav Petkov wrote:
> On Thu, May 14, 2015 at 06:17:28PM +0800, Huang Rui wrote:
> > At current processors, MWAITX cannot go to C1 state, but has faster
> > waiting exit speed. MWAITX is still at C0 but less power consumption.
> > So for now, I just want to set it optionally.
> > HW designer will do deeper low power state (C1, and more) in future
> > processors. At that time, we can enable it by default.
> 
> The only thing that matters IMO is which brings more power savings. If
> C1E is still better, we should keep it the default. We can still use
> MWAITX if someone boots with idle=mwait, as you say.
> 

Is C1E here you mentioned is waiting state that use mwaitx enters at
AMD platform? If yes, please see below comments:

Current processor:
Power saving: C0 < C1E (AMD) < C1
Performance: Halt < Mwait <= Mwaitx
Halt -> C1, and Mwaitx/Mwait -> C1E (AMD)

Consider about the balance between power consumption and performance,
so we want to expose the interface. And mwaitx has different opcode
with traditional mwait. Due to C1E (AMD) less power saving that real
C1, so you can think it still in C0 at current.

In furture processor:
HW designer would do Mwaitx -> C1 or deeper low power state.

BTW, could I expose it and send the patches to review?

Thanks,
Rui
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