linux-kernel - Re: [PATCH 2/2] cpuidle / menu: Return error code if there are no suitable states

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Message-ID: <1684440.TX1lbQesgE@vostro.rjw.lan>
Date:	Fri, 02 May 2014 14:20:18 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] cpuidle / menu: Return error code if there are no suitable states

On Friday, May 02, 2014 10:47:48 AM Daniel Lezcano wrote:
> On 04/30/2014 01:16 AM, Rafael J. Wysocki wrote:
> > On Tuesday, April 29, 2014 01:28:03 AM Rafael J. Wysocki wrote:
> >> On Monday, April 28, 2014 01:14:32 PM Daniel Lezcano wrote:
> >>> On 04/27/2014 02:55 PM, Rafael J. Wysocki wrote:
> 
> [ ... ]
> 
> > ---
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Subject: cpuidle / menu: Return (-1) if there are no suitable states
> >
> > If there is a PM QoS latency limit and all of the sufficiently shallow
> > C-states are disabled, the cpuidle menu governor returns 0 which on
> > some systems is CPUIDLE_DRIVER_STATE_START and shouldn't be returned
> > if that C-state has been disabled.
> >
> > Fix the issue by modifying the menu governor to return (-1) in such
> > situations.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >   drivers/cpuidle/governors/menu.c |    2 +-
> >   include/linux/cpuidle.h          |    2 ++
> >   2 files changed, 3 insertions(+), 1 deletion(-)
> >
> > Index: linux-pm/drivers/cpuidle/governors/menu.c
> > ===================================================================
> > --- linux-pm.orig/drivers/cpuidle/governors/menu.c
> > +++ linux-pm/drivers/cpuidle/governors/menu.c
> > @@ -296,7 +296,7 @@ static int menu_select(struct cpuidle_dr
> >   		data->needs_update = 0;
> >   	}
> >
> > -	data->last_state_idx = 0;
> > +	data->last_state_idx = CPUIDLE_DRIVER_STATE_START - 1;
> 
> In case of x86, CPUIDLE_DRIVER_STATE_START will be 1, so the select 
> function could return 0 even this one is disabled and this is not what 
> you want to happen, no ?

OK, so that's a choice.  We can choose to do the above or to return an error
code if the 0 state is disabled too.  The above is arguably simpler and
matches the idea that 0 is a "fallback" state on x86.

Of course, it also is confusing, because user space *can* set "disable" for
the 0 state on x86, but that actually has no effect today AFAICS.

I'm mostly worried about systems where CPUIDLE_DRIVER_STATE_START is 0
and where menu_select() explicitly checks "disabled" and then it returns
0 anyway if it cannot find any other suitable state.

In my opinion that needs to be made consistent, but I don't care too much about
which way as long as the change is not too intrusive.


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