Re: [PATCH v2 12/44] cpuidle,dt: Push RCU-idle into driver

Peter Zijlstra
2022-11-16T15:29:05Z

Sorry; things keep getting in the way of finishing this :/

As such, I need a bit of time to get on-track again..

On Tue, Oct 04, 2022 at 01:03:57PM +0200, Ulf Hansson wrote:

> > --- a/drivers/acpi/processor_idle.c
> > +++ b/drivers/acpi/processor_idle.c
> > @@ -1200,6 +1200,8 @@ static int acpi_processor_setup_lpi_stat
> > state->target_residency = lpi->min_residency;
> > if (lpi->arch_flags)
> > state->flags |= CPUIDLE_FLAG_TIMER_STOP;
> > + if (lpi->entry_method = ACPI_CSTATE_FFH)
> > + state->flags |= CPUIDLE_FLAG_RCU_IDLE;
>
> I assume the state index here will never be 0?
>
> If not, it may lead to that acpi_processor_ffh_lpi_enter() may trigger
> CPU_PM_CPU_IDLE_ENTER_PARAM() to call ct_cpuidle_enter|exit() for an
> idle-state that doesn't have the CPUIDLE_FLAG_RCU_IDLE bit set.

I'm not quite sure I see how. AFAICT this condition above implies
acpi_processor_ffh_lpi_enter() gets called, no?

Which in turn is an unconditional __CPU_PM_CPU_IDLE_ENTER() user, so
even if idx=0, it ends up in ct_idle_{enter,exit}().

>
> > state->enter = acpi_idle_lpi_enter;
> > drv->safe_state_index = i;
> > }
> > --- a/drivers/cpuidle/cpuidle-arm.c
> > +++ b/drivers/cpuidle/cpuidle-arm.c
> > @@ -53,6 +53,7 @@ static struct cpuidle_driver arm_idle_dr
> > * handler for idle state index 0.
> > */
> > .states[0] = {
> > + .flags = CPUIDLE_FLAG_RCU_IDLE,
>
> Comparing arm64 and arm32 idle-states/idle-drivers, the $subject
> series ends up setting the CPUIDLE_FLAG_RCU_IDLE for the ARM WFI idle
> state (state zero), but only for the arm64 and psci cases (mostly
> arm64). For arm32 we would need to update the ARM_CPUIDLE_WFI_STATE
> too, as that is what most arm32 idle-drivers are using. My point is,
> the code becomes a bit inconsistent.

True.

> Perhaps it's easier to avoid setting the CPUIDLE_FLAG_RCU_IDLE bit for
> all of the ARM WFI idle states, for both arm64 and arm32?

As per the below?

>
> > .enter = arm_enter_idle_state,
> > .exit_latency = 1,
> > .target_residency = 1,

> > --- a/include/linux/cpuidle.h
> > +++ b/include/linux/cpuidle.h
> > @@ -282,14 +282,18 @@ extern s64 cpuidle_governor_latency_req(
> > int __ret = 0; \
> > \
> > if (!idx) { \
> > + ct_idle_enter(); \
>
> According to my comment above, we should then drop these calls to
> ct_idle_enter and ct_idle_exit() here. Right?

Yes, if we ensure idx=0 never has RCU_IDLE set then these must be
removed.

> > cpu_do_idle(); \
> > + ct_idle_exit(); \
> > return idx; \
> > } \
> > \
> > if (!is_retention) \
> > __ret = cpu_pm_enter(); \
> > if (!__ret) { \
> > + ct_idle_enter(); \
> > __ret = low_level_idle_enter(state); \
> > + ct_idle_exit(); \
> > if (!is_retention) \
> > cpu_pm_exit(); \
> > } \
> >

So the basic premise is that everything that needs RCU inside the idle
callback must set CPUIDLE_FLAG_RCU_IDLE and by doing that promise to
call ct_idle_{enter,exit}() themselves.

Setting RCU_IDLE is required when there is RCU usage, however even if
there is no RCU usage, setting RCU_IDLE is fine, as long as
ct_idle_{enter,exit}() then get called.

So does the below (delta) look better to you?

--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -1218,7 +1218,7 @@ static int acpi_processor_setup_lpi_stat
state->target_residency = lpi->min_residency;
if (lpi->arch_flags)
state->flags |= CPUIDLE_FLAG_TIMER_STOP;
- if (lpi->entry_method = ACPI_CSTATE_FFH)
+ if (i != 0 && lpi->entry_method = ACPI_CSTATE_FFH)
state->flags |= CPUIDLE_FLAG_RCU_IDLE;
state->enter = acpi_idle_lpi_enter;
drv->safe_state_index = i;
--- a/drivers/cpuidle/cpuidle-arm.c
+++ b/drivers/cpuidle/cpuidle-arm.c
@@ -53,7 +53,7 @@ static struct cpuidle_driver arm_idle_dr
* handler for idle state index 0.
*/
.states[0] = {
- .flags = CPUIDLE_FLAG_RCU_IDLE,
+ .flags = 0,
.enter = arm_enter_idle_state,
.exit_latency = 1,
.target_residency = 1,
--- a/drivers/cpuidle/cpuidle-psci.c
+++ b/drivers/cpuidle/cpuidle-psci.c
@@ -357,7 +357,7 @@ static int psci_idle_init_cpu(struct dev
* PSCI idle states relies on architectural WFI to be represented as
* state index 0.
*/
- drv->states[0].flags = CPUIDLE_FLAG_RCU_IDLE;
+ drv->states[0].flags = 0;
drv->states[0].enter = psci_enter_idle_state;
drv->states[0].exit_latency = 1;
drv->states[0].target_residency = 1;
--- a/drivers/cpuidle/cpuidle-qcom-spm.c
+++ b/drivers/cpuidle/cpuidle-qcom-spm.c
@@ -72,7 +72,7 @@ static struct cpuidle_driver qcom_spm_id
.owner = THIS_MODULE,
.states[0] = {
.enter = spm_enter_idle_state,
- .flags = CPUIDLE_FLAG_RCU_IDLE,
+ .flags = 0,
.exit_latency = 1,
.target_residency = 1,
.power_usage = UINT_MAX,
--- a/drivers/cpuidle/cpuidle-riscv-sbi.c
+++ b/drivers/cpuidle/cpuidle-riscv-sbi.c
@@ -337,7 +337,7 @@ static int sbi_cpuidle_init_cpu(struct d
drv->cpumask = (struct cpumask *)cpumask_of(cpu);

/* RISC-V architectural WFI to be represented as state index 0. */
- drv->states[0].flags = CPUIDLE_FLAG_RCU_IDLE;
+ drv->states[0].flags = 0;
drv->states[0].enter = sbi_cpuidle_enter_state;
drv->states[0].exit_latency = 1;
drv->states[0].target_residency = 1;
--- a/include/linux/cpuidle.h
+++ b/include/linux/cpuidle.h
@@ -282,9 +282,7 @@ extern s64 cpuidle_governor_latency_req(
int __ret = 0; \
\
if (!idx) { \
- ct_idle_enter(); \
cpu_do_idle(); \
- ct_idle_exit(); \
return idx; \
} \
\