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

Nikunj Kela
2024-05-30T14:24:04Z

On 5/30/2024 1:15 AM, Ulf Hansson wrote:
> On Tue, 28 May 2024 at 21:56, Nikunj Kela <quic_nkela@quicinc.com> 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@linaro.org>
>>> ---
>>>
>>> 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