linux-kernel - Re: [PATCH 3/3] PM: Introduce Intel PowerClamp Driver

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Date:	Wed, 14 Nov 2012 19:22:14 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Linux PM <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Len Brown <len.brown@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Zhang Rui <rui.zhang@...el.com>, Rob Landley <rob@...dley.net>
Subject: Re: [PATCH 3/3] PM: Introduce Intel PowerClamp Driver

On Tue, Nov 13, 2012 at 06:59:06PM -0800, Arjan van de Ven wrote:
> On 11/13/2012 5:34 PM, Paul E. McKenney wrote:
> > On Tue, Nov 13, 2012 at 05:14:50PM -0800, Jacob Pan wrote:
> >> On Tue, 13 Nov 2012 16:08:54 -0800
> >> Arjan van de Ven <arjan@...ux.intel.com> wrote:
> >>
> >>>> I think I know, but I feel the need to ask anyway.  Why not tell
> >>>> RCU about the clamping?  
> >>>
> >>> I don't mind telling RCU, but what cannot happen is a bunch of CPU
> >>> time suddenly getting used (since that is the opposite of what is
> >>> needed at the specific point in time of going idle)
> > 
> > Another round of RCU_FAST_NO_HZ rework, you are asking for?  ;-)
> 
> well
> we can tell you we're about to mwait
> and we can tell you when we're done being idle.
> you could just do the actual work at that point, we don't care anymore ;-)
> just at the start of the mandated idle period we can't afford to have more
> jitter than we already have (which is more than I'd like, but it's manageable.
> More jitter means more performance hit, since during the time of the jitter, some cpus
> are forced idle, e.g. costing performance, without the actual big-step power savings
> kicking in yet....)

Fair enough -- but probably best to see what problems arise rather than
trying to guess too far ahead.  Who knows?  It might "just work".

> > If you are only having the system take 6-millisecond "vacations", probably
> 
> it's not all that different from running a while (1) loop for 6 msec inside
> a kernel thread.... other than the power level of course...

Well, a while (1) on all CPUs simultaneously, anyway.

							Thanx, Paul

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