Date:	Mon, 2 Mar 2015 10:08:23 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/2] drivers: cpuidle: minor suspend-to-idle fixes

On Sat, Feb 28, 2015 at 11:58:21PM +0000, Rafael J. Wysocki wrote:
> On Saturday, February 28, 2015 11:54:23 AM Lorenzo Pieralisi wrote:
> > On Fri, Feb 27, 2015 at 10:11:54PM +0000, Rafael J. Wysocki wrote:
> > > On Friday, February 27, 2015 10:00:00 AM Lorenzo Pieralisi wrote:
> > > > [CC'ed Preeti]
> > > > 
> > > > On Thu, Feb 26, 2015 at 11:37:54PM +0000, Rafael J. Wysocki wrote:
> > > > > Me versions of the two $subject patches follow.
> > > > 
> > > > Thank you. I am testing them and I have run into the following issue.
> > > > 
> > > > Starting with:
> > > > 
> > > > 3810631 ("PM / sleep: Re-implement suspend-to-idle handling")
> > > > 
> > > > the suspend-to-idle code path in the cpuidle_idle_call() bypasses
> > > > the CPUIDLE_FLAG_TIMER_STOP code path entirely.
> > > 
> > > Hmm, this looks like a mistake.  Sorry about that.
> > 
> > Thank you for looking into this !
> > 
> > > > Now, on most of
> > > > the current ARM platforms, the deepest idle state loses the tick device
> > > > context, therefore this means that going to idle through
> > > > suspend-to-idle becomes a brute force way of nuking the tick,
> > > > unless I am missing something here.
> > > > 
> > > > I am experiencing hangs on resume from suspend-to-idle when the broadcast
> > > > timer is the broadast-hrtimer (ie there is no HW broadcast timer in the
> > > > platform) and the deepest idle states lose the tick device context (ie
> > > > they are CPUIDLE_FLAG_TIMER_STOP), I hope Preeti can help me test this on
> > > > Power, still chasing the issue.
> > > > 
> > > > I could not reproduce the issue with a HW broadcast timer device.
> > > > 
> > > > Platform has deepest idle states that allow CPUs shutdown where the
> > > > local tick device is gone on entry, I am trying to provide you with a
> > > > backtrace, I need time to debug.
> > > > 
> > > > The question I have: is it safe to bypass the CPUIDLE_FLAG_TIMER_STOP
> > > > and related broadcast mode entry/exit in the suspend-to-idle path ?
> > > > 
> > > > I do not think it is, but I am asking.
> > > 
> > > It isn't in general, but it would be OK in the enter_freeze_proper() path
> > > where the tick is suspended anyway.
> > 
> > Entering state through enter_freeze() (ie adding the function pointer
> > in the idle states, on platforms where it is appropriate), therefore
> > freezing the tick solves the issue, so I think we should patch all ARM
> > drivers that can benefit from this interesting new feature.
> > 
> > > > I can "force" tick freeze by initializing the enter_freeze pointer
> > > > in the idle states (that's the next thing I will test), but still, for
> > > > platforms where that's not possible my question above is still valid.
> > > 
> > > Right.
> > > 
> > > Does the patch below help (on top of the previous ones)?
> > 
> > I need HW to test, I will do that first thing on Monday, and send
> > you the appropriate tags. I already put together a similar patch
> > and tested it yesterday, so I can say already that it solves the
> > problem, I will test your patch on Monday and get back to you
> > (patch 1 and 2 in this series already tested on a platform with no
> > CPUidle driver registered, and they work and fix the NULL drv
> > dereference issue).
> 
> Cool, thanks!
> 
> Below is a slightly cleaner version of the patch with a changelog.  Can you
> please test this one?
> 
> Rafael
> 
> 
> ---
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Subject: cpuidle / sleep: Use broadcast timer for states that stop local timer
> 
> Commit 381063133246 (PM / sleep: Re-implement suspend-to-idle handling)
> overlooked the fact that entering some sufficiently deep idle states
> by CPUs may cause their local timers to stop and in those cases it
> is necessary to switch over to a broadcase timer prior to entering
> the idle state.  If the cpuidle driver in use does not provide
> the new ->enter_freeze callback for any of the idle states, that
> problem affects suspend-to-idle too, but it is not taken into account
> after the changes made by commit 381063133246.
> 
> Fix that by moving the CPUIDLE_FLAG_TIMER_STOP flag check and the
> broadcast timer manipulations following it from cpuidle_idle_call()
> to cpuidle_enter() which will cause them to be done, if necessary,
> in the suspend-to-idle case as well as for runtime idle.
> 
> Fixes: 381063133246 (PM / sleep: Re-implement suspend-to-idle handling)
> Reported-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpuidle/cpuidle.c |   34 +++++++++++++++++++++++++++++-----
>  kernel/sched/idle.c       |   17 ++---------------
>  2 files changed, 31 insertions(+), 20 deletions(-)
> 
> Index: linux-pm/drivers/cpuidle/cpuidle.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/cpuidle.c
> +++ linux-pm/drivers/cpuidle/cpuidle.c
> @@ -230,15 +230,39 @@ int cpuidle_select(struct cpuidle_driver
>   * @dev:   the cpuidle device
>   * @index: the index in the idle state table
>   *
> - * Returns the index in the idle state, < 0 in case of error.
> - * The error code depends on the backend driver
> + * Returns the index in the idle state, < 0 in case of error.  -EBUSY is
> + * returned to indicate that the target state was temporarily inaccessible.
> + * The other error codes depend on the backend driver.
>   */
>  int cpuidle_enter(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>  		  int index)
>  {
> -	if (cpuidle_state_is_coupled(dev, drv, index))
> -		return cpuidle_enter_state_coupled(dev, drv, index);
> -	return cpuidle_enter_state(dev, drv, index);
> +	unsigned int broadcast;
> +	int ret;
> +
> +	broadcast = drv->states[index].flags & CPUIDLE_FLAG_TIMER_STOP;
> +
> +	/*
> +	 * Tell the time framework to switch to a broadcast timer
> +	 * because our local timer will be shutdown. If a local timer
> +	 * is used from another cpu as a broadcast timer, this call may
> +	 * fail if it is not available
> +	 */
> +	if (broadcast) {
> +		ret = clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER,
> +					 &dev->cpu);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	ret = cpuidle_state_is_coupled(dev, drv, index) ?
> +		cpuidle_enter_state_coupled(dev, drv, index) :
> +		cpuidle_enter_state(dev, drv, index);
> +
> +	if (broadcast)
> +		clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &dev->cpu);
> +
> +	return ret;

You have to check this return value in cpuidle_enter_freeze() too
otherwise we return to the idle thread on -EBUSY without
even executing arch_cpu_idle() and with IRQ disabled, code hits
the WARN_ON_ONCE line 180.

There are multiple ways of fixing the issue, either you check the
cpuidle_enter_freeze() return value (you add one) to cpuidle_idle_call()
to make code consistent with the cpuidle_idle_call "normal" idle
behaviour or you add the return value check in cpuidle_enter_freeze(),
I am fine both ways.

Thanks,
Lorenzo

>  }
>  
>  /**
> Index: linux-pm/kernel/sched/idle.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/idle.c
> +++ linux-pm/kernel/sched/idle.c
> @@ -81,7 +81,6 @@ static void cpuidle_idle_call(void)
>  	struct cpuidle_device *dev = __this_cpu_read(cpuidle_devices);
>  	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
>  	int next_state, entered_state;
> -	unsigned int broadcast;
>  
>  	/*
>  	 * Check if the idle task must be rescheduled. If it is the
> @@ -151,18 +150,6 @@ use_default:
>  		goto exit_idle;
>  	}
>  
> -	broadcast = drv->states[next_state].flags & CPUIDLE_FLAG_TIMER_STOP;
> -
> -	/*
> -	 * Tell the time framework to switch to a broadcast timer
> -	 * because our local timer will be shutdown. If a local timer
> -	 * is used from another cpu as a broadcast timer, this call may
> -	 * fail if it is not available
> -	 */
> -	if (broadcast &&
> -	    clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, &dev->cpu))
> -		goto use_default;
> -
>  	/* Take note of the planned idle state. */
>  	idle_set_state(this_rq(), &drv->states[next_state]);
>  
> @@ -176,8 +163,8 @@ use_default:
>  	/* The cpu is no longer idle or about to enter idle. */
>  	idle_set_state(this_rq(), NULL);
>  
> -	if (broadcast)
> -		clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &dev->cpu);
> +	if (entered_state == -EBUSY)
> +		goto use_default;
>  
>  	/*
>  	 * Give the governor an opportunity to reflect on the outcome
> 
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