linux-kernel - Re: [PATCH] Extend mwait idle to optimize away CAL and RES interrupts to an idle CPU -v1

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Message-ID: <20120227170527.GD2463@linux.vnet.ibm.com>
Date:	Mon, 27 Feb 2012 09:05:27 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Venki Pallipadi <venki@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Aaron Durbin <adurbin@...gle.com>,
	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Extend mwait idle to optimize away CAL and RES
 interrupts to an idle CPU -v1

On Mon, Feb 27, 2012 at 05:06:46PM +0800, Yong Zhang wrote:
> On Sat, Feb 25, 2012 at 05:32:53PM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 24, 2012 at 01:41:50PM +0800, Yong Zhang wrote:
> > > On Thu, Feb 23, 2012 at 11:34:11AM -0800, Venki Pallipadi wrote:
> > > > On Thu, Feb 23, 2012 at 1:30 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > > > Why not scheduler_ipi()?
> > > > 
> > > > Was trying to avoid irq_enter/exit. As the work here is done in idle
> > > > thread context, I though we could avoid enter/exit. 
> > > 
> > > It seems we could not.
> > > At least RCU need it, see commit c5d753a55, otherwise we will get
> > > warning like 'RCU used illegally from extended quiescent state!'
> > 
> > If the use is tracing, then Steven Rostedt's patchset plus use of his
> > _rcuidle() tracing variants handles this:
> > 
> > 	https://lkml.org/lkml/2012/2/7/231
> > 
> > If this is instead algorithmic use of RCU, a set of patches I have queued
> > up for 3.4 will be required.
> 
> scheduler_ipi() doing more than tracing. Will look at your patches :)

Ah!  The key question is whether or not the code in question is called
both from idle and from non-idle.  This will be easiest if the code is
called only from idle, in which case you should only need this one:

	https://lkml.org/lkml/2012/2/3/498

							Thanx, Paul

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