On Wed, 16 Nov 2022 at 16:29, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
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}().
Seems like I was overlooking something here, you are right, this
shouldn't really be a problem.
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.
Right, I was thinking that it could make sense to shrink the window
for users getting this wrong. In other words, we shouldn't set the
CPUIDLE_FLAG_RCU_IDLE unless we really need to.
And as I said, consistent behaviour is also nice to have.
So does the below (delta) look better to you?
Yes, it does!
Although, one minor comment below.
--- 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,
Nitpick: I don't think we need to explicitly clear the flag, as it
should already be zeroed by the compiler from its static declaration.
Right?
.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; \
} \
\
Kind regards
Uffe
