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SubjectRe: [RFC][PATCH 4/4] acpi: Take over RCU-idle for C3-BM idle
Date
On 2020-09-15 12:32 +0200, Peter Zijlstra wrote:

> 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>

I got modpost errors in 5.9-rc6 after this patch:

ERROR: modpost: "rcu_idle_enter" [drivers/acpi/processor.ko] undefined!
ERROR: modpost: "rcu_idle_exit" [drivers/acpi/processor.ko] undefined!

Reverting commit 1fecfdbb7acc made them go away. Notably my
configuration had CONFIG_ACPI_PROCESSOR=m, changing that
to CONFIG_ACPI_PROCESSOR=y let the build succeed as well.

Cheers,
Sven

> ---
> 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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