linux-kernel - Re: [PATCH v2 1/4] x86, mwaitt: add monitorx and mwaitx instruction

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Message-ID: <20150609170613.GI22105@pd.tnic>
Date:	Tue, 9 Jun 2015 19:06:14 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Xue, Ken" <Ken.Xue@....com>, Thomas Gleixner <tglx@...utronix.de>,
	"Li, Tony" <Tony.Li@....com>, "x86@...nel.org" <x86@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Aaron Lu <aaron.lu@...el.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Len Brown <lenb@...nel.org>, Huang Rui <ray.huang@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 1/4] x86, mwaitt: add monitorx and mwaitx instruction

On Tue, Jun 09, 2015 at 09:44:59AM -0700, Andy Lutomirski wrote:
> [1] For those who weren't bitten by this repeatedly, modern Intel CPUs
> (at least Sandy Bridge, anyway) will, by default, detect when all
> cores are in C1 or deeper, think to themselves "wow, the OS selected
> C1 -- it must want a very deep sleep indeed", and put the whole
> package into some kind of deep sleep state.  The subsequent wakeup
> takes tens of milliseconds.  Doing this in udelay would be awful.

That's a good point. Reportedly, the current MWAITX enters something
between C0 and C1 but the way I understood it, going forward, it will
enter deeper sleep states.

So for shallow C-states, your idle enter/exit latency is low enough but
I'd guess deeper states would be a problem.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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