Re: [RFC][PATCH 4/4] acpi: Take over RCU-idle for C3-BM idle

[Sven Joachim]

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@xxxxxxxxxxxxx>

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)