linux-kernel - Re: [PATCH v2 1/6] idle: move the cpuidle entry point to the generic idle loop

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Message-ID: <alpine.LFD.2.11.1401301056180.1652@knanqh.ubzr>
Date:	Thu, 30 Jan 2014 11:07:48 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Olof Johansson <olof@...om.net>,
	Russell King <linux@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/6] idle: move the cpuidle entry point to the generic
 idle loop

On Thu, 30 Jan 2014, Daniel Lezcano wrote:

> On 01/30/2014 06:28 AM, Nicolas Pitre wrote:
> > On Thu, 30 Jan 2014, Preeti U Murthy wrote:
> >
> > > Hi Nicolas,
> > >
> > > On 01/30/2014 02:01 AM, Nicolas Pitre wrote:
> > > > On Wed, 29 Jan 2014, Nicolas Pitre wrote:
> > > >
> > > > > In order to integrate cpuidle with the scheduler, we must have a
> > > > > better
> > > > > proximity in the core code with what cpuidle is doing and not delegate
> > > > > such interaction to arch code.
> > > > >
> > > > > Architectures implementing arch_cpu_idle() should simply enter
> > > > > a cheap idle mode in the absence of a proper cpuidle driver.
> > > > >
> > > > > Signed-off-by: Nicolas Pitre <nico@...aro.org>
> > > > > Acked-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> > > >
> > > > As mentioned in my reply to Olof's comment on patch #5/6, here's a new
> > > > version of this patch adding the safety local_irq_enable() to the core
> > > > code.
> > > >
> > > > ----- >8
> > > >
> > > > From: Nicolas Pitre <nicolas.pitre@...aro.org>
> > > > Subject: idle: move the cpuidle entry point to the generic idle loop
> > > >
> > > > In order to integrate cpuidle with the scheduler, we must have a better
> > > > proximity in the core code with what cpuidle is doing and not delegate
> > > > such interaction to arch code.
> > > >
> > > > Architectures implementing arch_cpu_idle() should simply enter
> > > > a cheap idle mode in the absence of a proper cpuidle driver.
> > > >
> > > > In both cases i.e. whether it is a cpuidle driver or the default
> > > > arch_cpu_idle(), the calling convention expects IRQs to be disabled
> > > > on entry and enabled on exit. There is a warning in place already but
> > > > let's add a forced IRQ enable here as well.  This will allow for
> > > > removing the forced IRQ enable some implementations do locally and
> > >
> > > Why would this patch allow for removing the forced IRQ enable that are
> > > being done on some archs in arch_cpu_idle()? Isn't this patch expecting
> > > the default arch_cpu_idle() to have re-enabled the interrupts after
> > > exiting from the default idle state? Its supposed to only catch faulty
> > > cpuidle drivers that haven't enabled IRQs on exit from idle state but
> > > are expected to have done so, isn't it?
> >
> > Exact.  However x86 currently does this:
> >
> >  if (cpuidle_idle_call())
> >          x86_idle();
> >  else
> >          local_irq_enable();
> >
> > So whenever cpuidle_idle_call() is successful then IRQs are
> > unconditionally enabled whether or not the underlying cpuidle driver has
> > properly done it or not.  And the reason is that some of the x86 cpuidle
> > do fail to enable IRQs before returning.
> >
> > So the idea is to get rid of this unconditional IRQ enabling and let the
> > core issue a warning instead (as well as enabling IRQs to allow the
> > system to run).
> 
> But what I don't get with your comment is the local_irq_enable is done from
> the cpuidle common framework in 'cpuidle_enter_state' it is not done from the
> arch specific backend cpuidle driver.

Oh well... This certainly means we'll have to clean this mess as some 
drivers do it on their own while some others don't.  Some drivers also 
loop on !need_resched() while some others simply return on the first 
interrupt.

> So the code above could be:
> 
> 	if (cpuidle_idle_call())
> 		x86_idle();
> 
> without the else section, this local_irq_enable is pointless. Or may be I
> missed something ?

A later patch removes it anyway.  But if it is really necessary to 
enable interrupts then the core will do it but with a warning now.


Nicolas
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