Message-ID: <20150112161453.GA25281@red-moon>
Date:	Mon, 12 Jan 2015 16:14:53 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Sören Brinkmann <soren.brinkmann@...inx.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH RESEND] PM / sleep: Fix racing timers

On Mon, Jan 12, 2015 at 03:55:05PM +0000, Sören Brinkmann wrote:
> On Mon, 2015-01-12 at 03:43PM +0000, Lorenzo Pieralisi wrote:
> > Hi Rafael, Soren,
> >
> > On Sun, Jan 11, 2015 at 11:20:36PM +0000, Rafael J. Wysocki wrote:
> > > On Friday, January 09, 2015 01:50:59 PM Sören Brinkmann wrote:
> > > > On Sat, 2014-11-08 at 03:56PM -0800, Sören Brinkmann wrote:
> > > > > Hi Rafael,
> > > > >
> > > > > On Thu, 2014-11-06 at 01:33AM +0100, Rafael J. Wysocki wrote:
> > > > > > On Thursday, October 02, 2014 09:01:15 AM Sören Brinkmann wrote:
> > > > > > > Hi Rafael,
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > Sorry for the huge delay.
> > > > > >
> > > > > > > On Tue, 2014-09-23 at 01:01AM +0200, Rafael J. Wysocki wrote:
> > > > > > > > On Monday, September 22, 2014 10:07:03 AM Soren Brinkmann wrote:
> > > > > > > > > On platforms that do not power off during suspend, successfully entering
> > > > > > > > > suspend races with timers.
> > > > > > > > >
> > > > > > > > > The race happening in a couple of location is:
> > > > > > > > >
> > > > > > > > >   1. disable IRQs               (e.g. arch_suspend_disable_irqs())
> > > > > > > > >      ...
> > > > > > > > >   2. syscore_suspend()
> > > > > > > > >       -> timekeeping_suspend()
> > > > > > > > >        -> clockevents_notify(SUSPEND)
> > > > > > > > >         -> tick_suspend()       (timers are turned off here)
> > > > > > > > >      ...
> > > > > > > > >   3. wfi                        (wait for wake-IRQ here)
> > > > > > > > >
> > > > > > > > > Between steps 1 and 2 the timers can still generate interrupts that are
> > > > > > > > > not handled and stay pending until step 3. That pending IRQ causes an
> > > > > > > > > immediate - spurious - wake.
> > > > > > > > >
> > > > > > > > > The solution is to move the clockevents suspend/resume notification
> > > > > > > > > out of the syscore_suspend step and explictly call them at the appropriate
> > > > > > > > > time in the suspend/hibernation paths. I.e. timers are suspend _before_
> > > > > > > > > IRQs get disabled. And accordingly in the resume path.
> > > > > > > > >
> > > > > > > > > Signed-off-by: Soren Brinkmann <soren.brinkmann@...inx.com>
> > > > > > > > > ---
> > > > > > > > > Hi,
> > > > > > > > >
> > > > > > > > > there was not a lot of discussion on the last submission. Just one comment from
> > > > > > > > > Rafael (https://lkml.org/lkml/2014/8/26/780), which - as I outlined in my
> > > > > > > > > response, does not apply, IMHO, since the platform does not re-enable
> > > > > > > > > interrupts.
> > > > > > > >
> > > > > > > > Well, you just don't agree with it.
> > > > > > > >
> > > > > > > > The problem with your approach is that timer interrupts aren't actually as
> > > > > > > > special as you think and any other IRQF_NO_SUSPEND interrupts would have caused
> > > > > > > > similar issues to appear under specific conditions.
> > > > > > > >
> > > > > > > > The solution I would suggest and that actually covers all IRQF_NO_SUSPEND
> > > > > > > > interrupts would be to use a wait_event() loop like the one in freeze_enter()
> > > > > > > > (on top of the current linux-next or the pm-genirq branch of linux-pm.git),
> > > > > > > > but wait for pm_abort_suspend to become true, to implement system suspend.
> > > > > > >
> > > > > > > sorry, it took me a while since I needed to get some dependencies ported
> > > > > > > to the pm-genirq base. Once I had that, it reproduced my original issue.
> > > > > > > So far so good. I then looked into finding a solution following your
> > > > > > > guidance. I'm not sure I really found what you had in mind, but below is
> > > > > > > what I came up with, which seems to do it.
> > > > > > > Please let me know how far off I am.
> > > > > > >
> > > > > > >     Thanks,
> > > > > > >     Sören
> > > > > > >
> > > > > > > -------8<------------------8<----------------8<----------------8<---------------
> > > > > > > diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
> > > > > > > index c2744b30d5d9..a4f9914571f1 100644
> > > > > > > --- a/drivers/base/power/wakeup.c
> > > > > > > +++ b/drivers/base/power/wakeup.c
> > > > > > > @@ -25,7 +25,7 @@
> > > > > > >  bool events_check_enabled __read_mostly;
> > > > > > >
> > > > > > >  /* If set and the system is suspending, terminate the suspend. */
> > > > > > > -static bool pm_abort_suspend __read_mostly;
> > > > > > > +bool pm_abort_suspend __read_mostly;
> > > > > > >
> > > > > > >  /*
> > > > > > >   * Combined counters of registered wakeup events and wakeup events in progress.
> > > > > > > diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
> > > > > > > index 6dadb25cb0d8..e6a6de8f76d0 100644
> > > > > > > --- a/kernel/power/suspend.c
> > > > > > > +++ b/kernel/power/suspend.c
> > > > > > > @@ -33,6 +33,7 @@
> > > > > > >
> > > > > > >  static const char *pm_labels[] = { "mem", "standby", "freeze", };
> > > > > > >  const char *pm_states[PM_SUSPEND_MAX];
> > > > > > > +extern bool pm_abort_suspend;
> > > > > > >
> > > > > > >  static const struct platform_suspend_ops *suspend_ops;
> > > > > > >  static const struct platform_freeze_ops *freeze_ops;
> > > > > > > @@ -294,25 +295,27 @@ static int suspend_enter(suspend_state_t state, bool *wakeup)
> > > > > > >     if (error || suspend_test(TEST_CPUS))
> > > > > > >             goto Enable_cpus;
> > > > > > >
> > > > > > > -   arch_suspend_disable_irqs();
> > > > > > > -   BUG_ON(!irqs_disabled());
> > > > > > > -
> > > > > > > -   error = syscore_suspend();
> > > > > > > -   if (!error) {
> > > > > > > -           *wakeup = pm_wakeup_pending();
> > > > > > > -           if (!(suspend_test(TEST_CORE) || *wakeup)) {
> > > > > > > -                   trace_suspend_resume(TPS("machine_suspend"),
> > > > > > > -                           state, true);
> > > > > > > -                   error = suspend_ops->enter(state);
> > > > > > > -                   trace_suspend_resume(TPS("machine_suspend"),
> > > > > > > -                           state, false);
> > > > > > > -                   events_check_enabled = false;
> > > > > > > +   while (!pm_abort_suspend) {
> > > > > >
> > > > > > That won't work in general, because pm_abort_suspend may not be set on some
> > > > > > platforms on wakeup.  It is only set if a wakeup interrupt triggers which
> > > > > > may not be the case on ACPI systems if the BIOS has woken up the system.
> > > > > >
> > > > > > But that could be addressed by making those platforms simply set pm_wakeup_pending
> > > > > > in their BIOS exit path.
> > > > > >
> > > > > > > +           arch_suspend_disable_irqs();
> > > > > > > +           BUG_ON(!irqs_disabled());
> > > > > > > +
> > > > > > > +           error = syscore_suspend();
> > > > > >
> > > > > > Also it shouldn't be necessary to do syscore_suspend()/syscore_resume() in
> > > > > > every iteration of the loop.
> > > > > >
> > > > > > > +           if (!error) {
> > > > > > > +                   *wakeup = pm_wakeup_pending();
> > > > > >
> > > > > > Plus pm_wakeup_pending() returns true if pm_abort_suspend is set
> > > > > >
> > > > > > > +                   if (!(suspend_test(TEST_CORE) || *wakeup)) {
> > > > > > > +                           trace_suspend_resume(TPS("machine_suspend"),
> > > > > > > +                                   state, true);
> > > > > >
> > > > > > Did you try to add the loop here instead of above?  Like:
> > > > > >
> > > > > >                       for (;;) {
> > > > > >                               *wakeup = pm_wakeup_pending();
> > > > > >                               if (*wakeup)
> > > > > >                                       break;
> > > > >
> > > > > I think, that doesn't work. I chose the start/end points of the loop
> > > > > to include the IRQ enable/disable calls. AFAICT, pm_abort_suspend is
> > > > > set in an ISR. Without enabling interrupts the abort condition of
> > > > > this loop never becomes true.
> > > >
> > > > Any further ideas how to resolve this?
> > >
> > > Sorry about the delay, lost track of this.
> > >
> > > You're right, the IRQ disabling/enabling needs to happen in the loop.
> > >
> > > So the direction of the patch looks OK, but it needs to ensure that pm_wakeup_pending
> > > is set properly by all platforms.  Also it should be sufficient to check
> > > pm_wakeup_pending() to detect wakeup.
> > >
> > > Have you tested the patch?
> >
> > Before considering this patch a solution, can I ask you to rewind
> > the discussion a bit since I have a question.
> >
> > I thought that "suspending" the tick through syscore meant shutting down
> > the respective clock_event_device, and that's how it is implemented in the
> > kernel.
> >
> > Now, do we expect a shutdown clock_event_device to still signal pending
> > IRQs ? I do not think that should be the case, at least that's not what
> > happens for eg arm arch timers - ie disabling them implicitly gates
> > the signal connected to the IRQ line.
> >
> > So the question is more related to the zynq platform and how their clock
> > event device (which is ?) is shutdown, and what's the correct behaviour we
> > are expecting.
> 
> As outlined in the commit message, there is a race condition in the core
> code. Looking at the timers is just fighting the symptoms.

I gathered there is a race condition between 1 and 2 in your code path.
What I am asking you is why are we getting a pending IRQ at step 3 when the
clock event device is supposed to be shutdown. My question is:

Should a clock event device in shutdown mode (ie disabled) still signal
IRQs to the interrupt controller (and consequently to the core) ?

It is for me to understand if that's the behaviour we are expecting.

> > FWIW, the problem here is not related to the simple wfi state on zynq,
> > even some other ARM platforms with power management capabilities would wake
> > up from the state entered by executing wfi (ie possibly through reset) if
> > there is a pending timer IRQ, the question is more "should the IRQ be
> > allowed to be there" instead IMHO.
> >
> > I still think that Stephen's query related to what timer is causing
> > the wake-up is worth investigating.
> 
> As I reported earlier, I see these spurious wakes with the cadence_ttc
> as well as the ARM twd timers.

I thought that a shutdown clock event device explicitly disables IRQ
assertion, that's why I am inquiring, I do not understand how this
can happen - how can you have a pending timer IRQ at step 3 above.

Thanks,
Lorenzo
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