linux-kernel - Re: [PATCH] PM / QoS: Fix device resume latency PM QoS

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Date:   Wed, 25 Oct 2017 18:41:26 -0700
From:   Ramesh Thomas <ramesh.thomas@...el.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Alex Shi <alex.shi@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH] PM / QoS: Fix device resume latency PM QoS

On 2017-10-25 at 18:28:01 +0200, Rafael J. Wysocki wrote:
> On Wed, Oct 25, 2017 at 9:27 AM, Ramesh Thomas <ramesh.thomas@...el.com> wrote:
> > On 2017-10-24 at 13:23:23 +0200, Rafael J. Wysocki wrote:
> >> On Tue, Oct 24, 2017 at 10:49 AM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >> > On Tuesday, October 24, 2017 7:54:09 AM CEST Ramesh Thomas wrote:
> >> >> On 2017-10-20 at 13:27:34 +0200, Rafael J. Wysocki wrote:
> >> >> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >> >> >
> >>
> >> [cut]
> >>
> >> >> > @@ -63,10 +60,14 @@ static bool default_suspend_ok(struct de
> >> >> >
> >> >> >     spin_unlock_irqrestore(&dev->power.lock, flags);
> >> >> >
> >> >> > -   if (constraint_ns < 0)
> >> >> > +   if (constraint_ns == 0)
> >> >> >             return false;
> >> >> >
> >> >> > -   constraint_ns *= NSEC_PER_USEC;
> >> >> > +   if (constraint_ns == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
> >> >> > +           constraint_ns = -1;
> >> >> > +   else
> >> >> > +           constraint_ns *= NSEC_PER_USEC;
> >> >> > +
> >> >> >     /*
> >> >> >      * We can walk the children without any additional locking, because
> >> >> >      * they all have been suspended at this point and their
> >> >> > @@ -76,14 +77,19 @@ static bool default_suspend_ok(struct de
> >> >> >             device_for_each_child(dev, &constraint_ns,
> >> >> >                                   dev_update_qos_constraint);
> >> >> >
> >> >> > -   if (constraint_ns > 0) {
> >> >> > -           constraint_ns -= td->suspend_latency_ns +
> >> >> > -                           td->resume_latency_ns;
> >> >> > -           if (constraint_ns == 0)
> >> >> > -                   return false;
> >> >> > +   if (constraint_ns < 0) {
> >> >> > +           /* The children have no constraints. */
> >> >> > +           td->effective_constraint_ns = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
> >> >> > +           td->cached_suspend_ok = true;
> >> >> > +   } else {
> >> >> > +           constraint_ns -= td->suspend_latency_ns + td->resume_latency_ns;
> >> >> > +           if (constraint_ns > 0) {
> >> >> > +                   td->effective_constraint_ns = constraint_ns;
> >> >> > +                   td->cached_suspend_ok = true;
> >> >> > +           } else {
> >> >> > +                   td->effective_constraint_ns = 0;
> >> >>
> >> >> Previously effective_constraint_ns was left as -1 if constraint_ns becomes 0
> >> >> Not sure if this change is intentional.
> >> >
> >> > Yes, it is.
> >> >
> >> >> I think at dev_update_qos_constraint, this can cause to skip call to
> >> >> dev_pm_qos_read_value.
> >> >
> >> > I need to double check that.
> >>
> >> If constraint_ns becomes 0 (or less) here, power cannot be removed
> >> from the device, because it would add an unacceptable latency.
> >>
> >> Thus effective_constraint_ns has to be 0 for it to indicate that
> >> situation.  If it was left at -1, it would mean "no requirement", but
> >> that wouldn't be correct.
> >>
> >
> > A negative value in effective_constraint_ns is used as trigger to read new
> > resume latency constraints.
> 
> I guess you mean in __default_power_down_ok(), right?

Yes.

> 
> That doesn't matter, because it covers the case when the device has
> never been runtime-suspended: it started in the "suspended" state and
> has never been made "active".
> 
> The case we are talking about is when default_suspend_ok() *was* run
> and it returned "true", or the device would not have been suspended,
> so __default_power_down_ok() would not have run for that domain at
> all.  In that case effective_constraint has to be positive anyway,
> because that is the only case when default_suspend_ok() returns
> "true".
> 
> It matters in default_suspend_ok() itself, however, where the
> constraints for the children are checked and -1 means "no
> restriction".  So it still looks like the patch needs to be improved,
> but that's because effective_constraint should not remain -1 if
> constraint_ns is 0 (which it still does in one case).

If you are referring to the place where it exits when constraint_ns == 0,
then I think it should be ok because it returns false there. Unless I
am missing something, the device would not suspend and neither the parent
nor __default_power_down_ok() would be referring to that value in that case.

Thanks,
Ramesh