linux-kernel - Re: [RFC PATCH RT] PM: runtime: avoid retry loops on RT

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Date:   Wed, 6 Oct 2021 19:05:50 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     John Keeping <john@...anate.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-rt-users@...r.kernel.org, Pavel Machek <pavel@....cz>,
        Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH RT] PM: runtime: avoid retry loops on RT

On Tue, Oct 5, 2021 at 7:17 PM John Keeping <john@...anate.com> wrote:
>
> On Tue, 5 Oct 2021 18:38:27 +0200
> "Rafael J. Wysocki" <rafael@...nel.org> wrote:
>
> > On Tue, Oct 5, 2021 at 6:14 PM John Keeping <john@...anate.com> wrote:
> > >
> > > With PREEMPT_RT spin_unlock() is identical to spin_unlock_irq() so there
> > > is no reason to have a special case using the former.  Furthermore,
> > > spin_unlock() enables preemption meaning that a task in RESUMING or
> > > SUSPENDING state may be preempted by a higher priority task running
> > > pm_runtime_get_sync() leading to a livelock.
> > >
> > > Use the non-irq_safe path for all waiting so that the waiting task will
> > > block.
> > >
> > > Note that this changes only the waiting behaviour of irq_safe, other
> > > uses are left unchanged so that the parent device always remains active
> > > in the same way as !RT.
> > >
> > > Signed-off-by: John Keeping <john@...anate.com>
> >
> > So basically, the idea is that the irq_safe flag should have no effect
> > when CONFIG_PREEMPT_RT is set, right?
> >
> > Wouldn't it be cleaner to make it not present at all in that case?
>
> Yes, just replacing pm_runtime_irq_safe() with an empty function would
> also fix it, but I'm not sure if that will have unexpected effects from
> the parent device suspending/resuming, especially in terms of latency
> for handling interrupts.

Well, the code as is doesn't work with CONFIG_PREEMPT_RT set anyway in general.

Also this is not just pm_runtime_irq_safe(), but every access to this
flag (and there's more  of them than just the ones changed below).

What about putting the flag under #ifdef CONFIG_PREEMPT_RT and
providing read/write accessor helpers for it that will be empty in
RT-enabled kernels?

> > > ---
> > >  drivers/base/power/runtime.c | 9 +++++----
> > >  1 file changed, 5 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > > index 96972d5f6ef3..5e0d349fab4e 100644
> > > --- a/drivers/base/power/runtime.c
> > > +++ b/drivers/base/power/runtime.c
> > > @@ -347,8 +347,9 @@ static int __rpm_callback(int (*cb)(struct device *), struct device *dev)
> > >  {
> > >         int retval = 0, idx;
> > >         bool use_links = dev->power.links_count > 0;
> > > +       bool irq_safe = dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT);
> > >
> > > -       if (dev->power.irq_safe) {
> > > +       if (irq_safe) {
> > >                 spin_unlock(&dev->power.lock);
> > >         } else {
> > >                 spin_unlock_irq(&dev->power.lock);
> > > @@ -376,7 +377,7 @@ static int __rpm_callback(int (*cb)(struct device *), struct device *dev)
> > >         if (cb)
> > >                 retval = cb(dev);
> > >
> > > -       if (dev->power.irq_safe) {
> > > +       if (irq_safe) {
> > >                 spin_lock(&dev->power.lock);
> > >         } else {
> > >                 /*
> > > @@ -596,7 +597,7 @@ static int rpm_suspend(struct device *dev, int rpmflags)
> > >                         goto out;
> > >                 }
> > >
> > > -               if (dev->power.irq_safe) {
> > > +               if (dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > >                         spin_unlock(&dev->power.lock);
> > >
> > >                         cpu_relax();
> > > @@ -777,7 +778,7 @@ static int rpm_resume(struct device *dev, int rpmflags)
> > >                         goto out;
> > >                 }
> > >
> > > -               if (dev->power.irq_safe) {
> > > +               if (dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > >                         spin_unlock(&dev->power.lock);
> > >
> > >                         cpu_relax();
> > > --
> > > 2.33.0
> > >
>