Re: [RFT][PATCH 1/2] PM / domains: Rework governor code to be more consistent

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Nov 3, 2017 at 8:05 AM, Ramesh Thomas <ramesh.thomas@intel.com> wrote:
> On 2017-11-02 at 00:01:50 +0100, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>
>> The genpd governor currently uses negative PM QoS values to indicate
>> the "no suspend" condition and 0 as "no restriction", but it doesn't
>> use them consistently.  Moreover, it tries to refresh QoS values for
>> already suspended devices in a quite questionable way.
>>
>> For the above reasons, rework it to be a bit more consistent.
>>
>> First off, note that dev_pm_qos_read_value() in
>> dev_update_qos_constraint() and __default_power_down_ok() is
>> evaluated for devices in suspend.  Moreover, that only happens if the
>> effective_constraint_ns value for them is negative (meaning "no
>> suspend").  It is not evaluated in any other cases, so effectively
>> the QoS values are only updated for devices in suspend that should
>> not have been suspended in the first place.  In all of the other
>> cases, the QoS values taken into account are the effective ones from
>> the time before the device has been suspended, so generally devices
>> need to be resumed and suspended again for new QoS values to take
>> effect anyway.  Thus evaluating dev_update_qos_constraint() in
>> those two places doesn't make sense at all, so drop it.
>>
>> Second, initialize effective_constraint_ns to 0 ("no constraint")
>> rather than to (-1) ("no suspend"), which makes more sense in
>> general and in case effective_constraint_ns is never updated
>> (the device is in suspend all the time or it is never suspended)
>> it doesn't affect the device's parent and so on.
>>
>> Finally, rework default_suspend_ok() to explicitly handle the
>> "no restriction" and "no suspend" special cases.
>>
>> Also add WARN_ON() around checks that should never trigger.

First, please note the above sentence.

>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> ---
>>  drivers/base/power/domain.c          |    2 -
>>  drivers/base/power/domain_governor.c |   56 ++++++++++++++++++++---------------
>>  2 files changed, 33 insertions(+), 25 deletions(-)
>>
>> Index: linux-pm/drivers/base/power/domain.c
>> ===================================================================
>> --- linux-pm.orig/drivers/base/power/domain.c
>> +++ linux-pm/drivers/base/power/domain.c
>> @@ -1331,7 +1331,7 @@ static struct generic_pm_domain_data *ge
>>
>>       gpd_data->base.dev = dev;
>>       gpd_data->td.constraint_changed = true;
>> -     gpd_data->td.effective_constraint_ns = -1;
>> +     gpd_data->td.effective_constraint_ns = 0;
>>       gpd_data->nb.notifier_call = genpd_dev_pm_qos_notifier;
>>
>>       spin_lock_irq(&dev->power.lock);
>> Index: linux-pm/drivers/base/power/domain_governor.c
>> ===================================================================
>> --- linux-pm.orig/drivers/base/power/domain_governor.c
>> +++ linux-pm/drivers/base/power/domain_governor.c
>> @@ -14,22 +14,22 @@
>>  static int dev_update_qos_constraint(struct device *dev, void *data)
>>  {
>>       s64 *constraint_ns_p = data;
>> -     s32 constraint_ns = -1;
>> -
>> -     if (dev->power.subsys_data && dev->power.subsys_data->domain_data)
>> -             constraint_ns = dev_gpd_data(dev)->td.effective_constraint_ns;
>> +     s64 constraint_ns;
>>
>> -     if (constraint_ns < 0) {
>> -             constraint_ns = dev_pm_qos_read_value(dev);
>> -             constraint_ns *= NSEC_PER_USEC;
>> -     }
>> -     if (constraint_ns == 0)
>> +     if (!dev->power.subsys_data || !dev->power.subsys_data->domain_data)
>>               return 0;
>>
>>       /*
>> -      * constraint_ns cannot be negative here, because the device has been
>> -      * suspended.
>> +      * Only take suspend-time QoS constraints of devices into account,
>> +      * because constraints updated after the device has been suspended are
>> +      * not guaranteed to be taken into account anyway.  In order for them
>> +      * to take effect, the device has to be resumed and suspended again.
>>        */
>> +     constraint_ns = dev_gpd_data(dev)->td.effective_constraint_ns;
>> +     /* 0 means "no constraint" */
>> +     if (constraint_ns == 0)
>> +             return 0;
>> +
>>       if (constraint_ns < *constraint_ns_p || *constraint_ns_p == 0)
>>               *constraint_ns_p = constraint_ns;
>>
>> @@ -76,14 +76,26 @@ static bool default_suspend_ok(struct de
>>               device_for_each_child(dev, &constraint_ns,
>>                                     dev_update_qos_constraint);
>>
>> -     if (constraint_ns > 0) {
>> +     if (constraint_ns == 0) {
>> +             /* "No restriction", so the device is allowed to suspend. */
>> +             td->effective_constraint_ns = 0;
>> +             td->cached_suspend_ok = true;
>> +     } else if (WARN_ON(constraint_ns < 0)) {
>
> Should never comer here. Child should not have suspended.
> Should be a fatal bug. This else block probably is not necessary.

Right.

>> +             /*
>> +              * effective_constraint_ns is negative already and
>> +              * cached_suspend_ok is false, so return right away.
>> +              */
>> +             return false;
>> +     } else {
>>               constraint_ns -= td->suspend_latency_ns +
>>                               td->resume_latency_ns;
>> -             if (constraint_ns == 0)
>> +             /* Again, if negative or zero, returning right away is fine. */
>> +             if (constraint_ns <= 0)
>>                       return false;
>> +
>> +             td->effective_constraint_ns = constraint_ns;
>> +             td->cached_suspend_ok = true;
>>       }
>> -     td->effective_constraint_ns = constraint_ns;
>> -     td->cached_suspend_ok = constraint_ns >= 0;
>>
>>       /*
>>        * The children have been suspended already, so we don't need to take
>> @@ -144,18 +156,14 @@ static bool __default_power_down_ok(stru
>>                */
>>               td = &to_gpd_data(pdd)->td;
>>               constraint_ns = td->effective_constraint_ns;
>> -             /* default_suspend_ok() need not be called before us. */
>> -             if (constraint_ns < 0) {
>> -                     constraint_ns = dev_pm_qos_read_value(pdd->dev);
>> -                     constraint_ns *= NSEC_PER_USEC;
>> -             }
>> +             /* Negative values mean "no suspend at all" */
>> +             if (WARN_ON(constraint_ns < 0))
>> +                     return false;
>
> Cannot come here if was not suspended. Looks like a redundant check.

Right, this and the above *are* probably redundant.  That's why
there's WARN_ON() here and in the other place.

Thanks,
Rafael