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: <20120228071255.GD1112@zhy>
Date:	Tue, 28 Feb 2012 15:12:55 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 09:05:27AM -0800, Paul E. McKenney wrote:
> 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.

In fact before this patch from Venki, the only call site of scheduler_ipi()
is resched irq handler. Then Venki introduce __scheduler_ipi()(which avoid
irq_enter()/irq_exit()) into cpu_idle(). So the answer is yes.

But when I was testing this patch, I didn't see explicit warning on
illegal rcu usage. The reason maybe 1) there are no much rcu dereference
in scheduler_ipi(), but we indeed do tracing in it; 2) rq->lock provide
some kind of protection.
Maybe I'm overstraining, but it is potential danger.

But anyway, it's not an issue anymore since Venki removed __scheduler_ipi()
in his latest version.

> 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

Hmm... Yeah, RCU_NONIDLE() could survive IMHO :)

Thanks,
Yong
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