linux-kernel - Re: [PATCH V3] cpuidle: Handle tick_broadcast

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Message-ID: <3161640.llJtBoKCBr@vostro.rjw.lan>
Date:	Sat, 09 May 2015 22:33:05 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	peterz@...radead.org, tglx@...utronix.de,
	rafael.j.wysocki@...el.com, daniel.lezcano@...aro.org,
	rlippert@...gle.com, linux-pm@...r.kernel.org,
	linus.walleij@...aro.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com, sudeep.holla@....com,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V3] cpuidle: Handle tick_broadcast_enter() failure gracefully

On Saturday, May 09, 2015 10:11:41 PM Rafael J. Wysocki wrote:
> On Saturday, May 09, 2015 11:19:16 AM Preeti U Murthy wrote:
> > Hi Rafael,
> > 
> > On 05/08/2015 07:48 PM, Rafael J. Wysocki wrote:
> 
> [cut]
> 
> > >>  
> > >> +	/* Take note of the planned idle state. */
> > >> +	idle_set_state(smp_processor_id(), target_state);
> > > 
> > > And I wouldn't do this either.
> > > 
> > > The behavior here is pretty much as though the driver demoted the state chosen
> > > by the governor and we don't call idle_set_state() again in those cases.
> > 
> > Why is this wrong?
> 
> It is not "wrong", but incomplete, because demotions done by the cpuidle driver
> should also be taken into account in the same way.
> 
> But I'm seeing that the recent patch of mine that made cpuidle_enter_state()
> call default_idle_call() was a mistake, because it might confuse find_idlest_cpu()
> significantly as to what state the CPU is in.  I'll drop that one for now.

OK, done.

So after I've dropped it I think we need to do three things:
(1) Move the idle_set_state() calls to cpuidle_enter_state().
(2) Make cpuidle_enter_state() call default_idle_call() again, but this time
    do that *before* it has called idle_set_state() for target_state.
(3) Introduce demotion as per my last patch.

Let me cut patches for that.


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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