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Subject[RFC][PATCH 4/4] acpi: Take over RCU-idle for C3-BM idle
The C3 BusMaster idle code takes lock in a number of places, some deep
inside the ACPI code. Instead of wrapping it all in RCU_NONIDLE, have
the driver take over RCU-idle duty and avoid flipping RCU state back
and forth a lot.

( by marking 'C3 && bm_check' as RCU_IDLE, we _must_ call enter_bm() for
that combination, otherwise we'll loose RCU-idle, this requires
shuffling some code around )

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
drivers/acpi/processor_idle.c | 69 +++++++++++++++++++++++++++++-------------
1 file changed, 49 insertions(+), 20 deletions(-)

--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -558,22 +558,43 @@ static DEFINE_RAW_SPINLOCK(c3_lock);

/**
* acpi_idle_enter_bm - enters C3 with proper BM handling
+ * @drv: cpuidle driver
* @pr: Target processor
* @cx: Target state context
+ * @index: index of target state
*/
-static void acpi_idle_enter_bm(struct acpi_processor *pr,
- struct acpi_processor_cx *cx)
+static int acpi_idle_enter_bm(struct cpuidle_driver *drv,
+ struct acpi_processor *pr,
+ struct acpi_processor_cx *cx,
+ int index)
{
+ static struct acpi_processor_cx safe_cx = {
+ .entry_method = ACPI_CSTATE_HALT,
+ };
+
/*
* disable bus master
* bm_check implies we need ARB_DIS
* bm_control implies whether we can do ARB_DIS
*
- * That leaves a case where bm_check is set and bm_control is
- * not set. In that case we cannot do much, we enter C3
- * without doing anything.
+ * That leaves a case where bm_check is set and bm_control is not set.
+ * In that case we cannot do much, we enter C3 without doing anything.
*/
- if (pr->flags.bm_control) {
+ bool dis_bm = pr->flags.bm_control;
+
+ /* If we can skip BM, demote to a safe state. */
+ if (!cx->bm_sts_skip && acpi_idle_bm_check()) {
+ dis_bm = false;
+ index = drv->safe_state_index;
+ if (index >= 0) {
+ cx = this_cpu_read(acpi_cstate[index]);
+ } else {
+ cx = &safe_cx;
+ index = -EBUSY;
+ }
+ }
+
+ if (dis_bm) {
raw_spin_lock(&c3_lock);
c3_cpu_count++;
/* Disable bus master arbitration when all CPUs are in C3 */
@@ -582,15 +603,21 @@ static void acpi_idle_enter_bm(struct ac
raw_spin_unlock(&c3_lock);
}

+ rcu_idle_enter();
+
acpi_idle_do_entry(cx);

+ rcu_idle_exit();
+
/* Re-enable bus master arbitration */
- if (pr->flags.bm_control) {
+ if (dis_bm) {
raw_spin_lock(&c3_lock);
acpi_write_bit_register(ACPI_BITREG_ARB_DISABLE, 0);
c3_cpu_count--;
raw_spin_unlock(&c3_lock);
}
+
+ return index;
}

static int acpi_idle_enter(struct cpuidle_device *dev,
@@ -604,20 +631,13 @@ static int acpi_idle_enter(struct cpuidl
return -EINVAL;

if (cx->type != ACPI_STATE_C1) {
+ if (cx->type == ACPI_STATE_C3 && pr->flags.bm_check)
+ return acpi_idle_enter_bm(drv, pr, cx, index);
+
+ /* C2 to C1 demotion. */
if (acpi_idle_fallback_to_c1(pr) && num_online_cpus() > 1) {
index = ACPI_IDLE_STATE_START;
cx = per_cpu(acpi_cstate[index], dev->cpu);
- } else if (cx->type == ACPI_STATE_C3 && pr->flags.bm_check) {
- if (cx->bm_sts_skip || !acpi_idle_bm_check()) {
- acpi_idle_enter_bm(pr, cx);
- return index;
- } else if (drv->safe_state_index >= 0) {
- index = drv->safe_state_index;
- cx = per_cpu(acpi_cstate[index], dev->cpu);
- } else {
- acpi_safe_halt();
- return -EBUSY;
- }
}
}

@@ -641,7 +661,13 @@ static int acpi_idle_enter_s2idle(struct
return 0;

if (pr->flags.bm_check) {
- acpi_idle_enter_bm(pr, cx);
+ u8 bm_sts_skip = cx->bm_sts_skip;
+
+ /* Don't check BM_STS, do an unconditional ARB_DIS for S2IDLE */
+ cx->bm_sts_skip = 1;
+ acpi_idle_enter_bm(drv, pr, cx, index);
+ cx->bm_sts_skip = bm_sts_skip;
+
return 0;
} else {
ACPI_FLUSH_CPU_CACHE();
@@ -674,8 +700,11 @@ static int acpi_processor_setup_cpuidle_
if (lapic_timer_needs_broadcast(pr, cx))
state->flags |= CPUIDLE_FLAG_TIMER_STOP;

- if (cx->type == ACPI_STATE_C3)
+ if (cx->type == ACPI_STATE_C3) {
state->flags |= CPUIDLE_FLAG_TLB_FLUSHED;
+ if (pr->flags.bm_check)
+ state->flags |= CPUIDLE_FLAG_RCU_IDLE;
+ }

count++;
if (count == CPUIDLE_STATE_MAX)

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