Re: [PATCH v2 1/7] pmdomain: core: Enable s2idle for CPU PM domains on PREEMPT_RT

[Nikunj Kela]

On 5/30/2024 1:15 AM, Ulf Hansson wrote:
> On Tue, 28 May 2024 at 21:56, Nikunj Kela <quic_nkela@xxxxxxxxxxx> wrote:
>>
>> On 5/27/2024 7:25 AM, Ulf Hansson wrote:
>>> To allow a genpd provider for a CPU PM domain to enter a domain-idle-state
>>> during s2idle on a PREEMPT_RT based configuration, we can't use the regular
>>> spinlock, as they are turned into sleepable locks on PREEMPT_RT.
>>>
>>> To address this problem, let's convert into using the raw spinlock, but
>>> only for genpd providers that have the GENPD_FLAG_CPU_DOMAIN bit set. In
>>> this way, the lock can still be acquired/released in atomic context, which
>>> is needed in the idle-path for PREEMPT_RT.
>>>
>>> Do note that the genpd power-on/off notifiers may also be fired during
>>> s2idle, but these are already prepared for PREEMPT_RT as they are based on
>>> the raw notifiers. However, consumers of them may need to adopt accordingly
>>> to work properly on PREEMPT_RT.
>>>
>>> Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
>>> ---
>>>
>>> Changes in v2:
>>>       - None.
>>>
>>> ---
>>>  drivers/pmdomain/core.c   | 47 ++++++++++++++++++++++++++++++++++++++-
>>>  include/linux/pm_domain.h |  5 ++++-
>>>  2 files changed, 50 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/pmdomain/core.c b/drivers/pmdomain/core.c
>>> index 623d15b68707..072e6bdb6ee6 100644
>>> --- a/drivers/pmdomain/core.c
>>> +++ b/drivers/pmdomain/core.c
>>> @@ -117,6 +117,48 @@ static const struct genpd_lock_ops genpd_spin_ops = {
>>>       .unlock = genpd_unlock_spin,
>>>  };
>>>
>>> +static void genpd_lock_raw_spin(struct generic_pm_domain *genpd)
>>> +     __acquires(&genpd->raw_slock)
>>> +{
>>> +     unsigned long flags;
>>> +
>>> +     raw_spin_lock_irqsave(&genpd->raw_slock, flags);
>>> +     genpd->raw_lock_flags = flags;
>>> +}
>>> +
>>> +static void genpd_lock_nested_raw_spin(struct generic_pm_domain *genpd,
>>> +                                     int depth)
>>> +     __acquires(&genpd->raw_slock)
>>> +{
>>> +     unsigned long flags;
>>> +
>>> +     raw_spin_lock_irqsave_nested(&genpd->raw_slock, flags, depth);
>>> +     genpd->raw_lock_flags = flags;
>>> +}
>>> +
>>> +static int genpd_lock_interruptible_raw_spin(struct generic_pm_domain *genpd)
>>> +     __acquires(&genpd->raw_slock)
>>> +{
>>> +     unsigned long flags;
>>> +
>>> +     raw_spin_lock_irqsave(&genpd->raw_slock, flags);
>>> +     genpd->raw_lock_flags = flags;
>>> +     return 0;
>>> +}
>>> +
>>> +static void genpd_unlock_raw_spin(struct generic_pm_domain *genpd)
>>> +     __releases(&genpd->raw_slock)
>>> +{
>>> +     raw_spin_unlock_irqrestore(&genpd->raw_slock, genpd->raw_lock_flags);
>>> +}
>>> +
>>> +static const struct genpd_lock_ops genpd_raw_spin_ops = {
>>> +     .lock = genpd_lock_raw_spin,
>>> +     .lock_nested = genpd_lock_nested_raw_spin,
>>> +     .lock_interruptible = genpd_lock_interruptible_raw_spin,
>>> +     .unlock = genpd_unlock_raw_spin,
>>> +};
>>> +
>>>  #define genpd_lock(p)                        p->lock_ops->lock(p)
>>>  #define genpd_lock_nested(p, d)              p->lock_ops->lock_nested(p, d)
>>>  #define genpd_lock_interruptible(p)  p->lock_ops->lock_interruptible(p)
>>> @@ -2079,7 +2121,10 @@ static void genpd_free_data(struct generic_pm_domain *genpd)
>>>
>>>  static void genpd_lock_init(struct generic_pm_domain *genpd)
>>>  {
>>> -     if (genpd->flags & GENPD_FLAG_IRQ_SAFE) {
>>> +     if (genpd->flags & GENPD_FLAG_CPU_DOMAIN) {
>>> +             raw_spin_lock_init(&genpd->raw_slock);
>>> +             genpd->lock_ops = &genpd_raw_spin_ops;
>>> +     } else if (genpd->flags & GENPD_FLAG_IRQ_SAFE) {
>> Hi Ulf, though you are targeting only CPU domains for now, I wonder if
>> FLAG_IRQ_SAFE will be a better choice?  The description of the flag says
>> it is safe for atomic context which won't be the case for PREEMPT_RT?
> You have a point!
>
> However, we also need to limit the use of raw spinlocks, from
> PREEMPT_RT point of view. In other words, just because a genpd
> provider is capable of executing its callbacks in atomic context,
> doesn't always mean that it should use raw spinlocks too.

Got it! Thanks. Maybe in future, if there is a need, a new GENPD FLAG
for RT, something like GENPD_FLAG_IRQ_SAFE_RT, can be added to address this.


>
> [...]
>
> Kind regards
> Uffe