[PATCH 1/9] intel_idle: Simplify LAPIC timer reliability checks

Rafael J. Wysocki
2020-02-13T22:05:13Z

The lapic_timer_always_reliable variable really takes only two values
and some arithmetic in intel_idle() related to comparing it with the
target C-state's MWAIT hint value is unnecessary.

Simplify the code by replacing lapic_timer_always_reliable with
a bool variable lapic_timer_always_reliable and dropping the
LAPIC_TIMER_ALWAYS_RELIABLE symbol along with the excess
computations in intel_idle().

While at it, add a comment explaining the branch taken in intel_idle()
if the LAPIC timer is only reliable in C1 and modify the related debug
message in intel_idle_init() accordingly (the modification of this
message in the only expected functional impact of the change made
here).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
drivers/idle/intel_idle.c | 27 +++++++++++++--------------
1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index d55606608ac8..8d66efc53b89 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -66,10 +66,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
static unsigned int disabled_states_mask;

static unsigned int mwait_substates;
-
-#define LAPIC_TIMER_ALWAYS_RELIABLE 0xFFFFFFFF
-/* Reliable LAPIC Timer States, bit 1 for C1 etc. */
-static unsigned int lapic_timer_reliable_states = (1 << 1); /* Default to only C1 */
+static bool lapic_timer_always_reliable;

struct idle_cpu {
struct cpuidle_state *state_table;
@@ -908,7 +905,6 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
unsigned long ecx = 1; /* break on interrupt flag */
struct cpuidle_state *state = &drv->states[index];
unsigned long eax = flg2MWAIT(state->flags);
- unsigned int cstate;
bool uninitialized_var(tick);
int cpu = smp_processor_id();

@@ -919,13 +915,16 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
if (state->flags & CPUIDLE_FLAG_TLB_FLUSHED)
leave_mm(cpu);

- if (!static_cpu_has(X86_FEATURE_ARAT)) {
- cstate = (((eax) >> MWAIT_SUBSTATE_SIZE) &
- MWAIT_CSTATE_MASK) + 1;
- tick = false;
- if (!(lapic_timer_reliable_states & (1 << (cstate)))) {
+ if (!static_cpu_has(X86_FEATURE_ARAT) && !lapic_timer_always_reliable) {
+ /*
+ * Switch over to one-shot tick broadcast if the target C-state
+ * is deeper than C1.
+ */
+ if ((eax >> MWAIT_SUBSTATE_SIZE) & MWAIT_CSTATE_MASK) {
tick = true;
tick_broadcast_enter();
+ } else {
+ tick = false;
}
}

@@ -1555,7 +1554,7 @@ static int intel_idle_cpu_online(unsigned int cpu)
{
struct cpuidle_device *dev;

- if (lapic_timer_reliable_states != LAPIC_TIMER_ALWAYS_RELIABLE)
+ if (!lapic_timer_always_reliable)
tick_broadcast_enable();

/*
@@ -1647,15 +1646,15 @@ static int __init intel_idle_init(void)
}

if (boot_cpu_has(X86_FEATURE_ARAT)) /* Always Reliable APIC Timer */
- lapic_timer_reliable_states = LAPIC_TIMER_ALWAYS_RELIABLE;
+ lapic_timer_always_reliable = true;

retval = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "idle/intel:online",
intel_idle_cpu_online, NULL);
if (retval < 0)
goto hp_setup_fail;

- pr_debug("lapic_timer_reliable_states 0x%x\n",
- lapic_timer_reliable_states);
+ pr_debug("Local APIC timer is reliable in %s\n",
+ lapic_timer_always_reliable ? "all C-states" : "C1");

return 0;

--
2.16.4