Re: [PATCH] ACPI: scan: Reduce overhead related to devices with dependencies

[Hans de Goede]

Hi Rafael,

On 5/15/23 18:30, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> Notice that all of the objects for which the acpi_scan_check_dep()
> return value is greater than 0 are present in acpi_dep_list as consumers
> (there may be multiple entries for one object, but that is not a
> problem), so after carrying out the initial ACPI namespace walk in which
> devices with dependencies are skipped, acpi_bus_scan() can simply walk
> acpi_dep_list and enumerate all of the unique consumer objects from
> there and their descendants instead of walking the entire target branch
> of the ACPI namespace and looking for device objects that have not been
> enumerated yet in it.
> 
> Because walking acpi_dep_list is generally less overhead than walking
> the entire ACPI namespace, use the observation above to reduce the
> system initialization overhead related to ACPI, which is particularly
> important on large systems.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>  drivers/acpi/scan.c     |   79 +++++++++++++++++++++++++++++++++++-------------
>  include/acpi/acpi_bus.h |    2 +
>  2 files changed, 61 insertions(+), 20 deletions(-)
> 
> Index: linux-pm/include/acpi/acpi_bus.h
> ===================================================================
> --- linux-pm.orig/include/acpi/acpi_bus.h
> +++ linux-pm/include/acpi/acpi_bus.h
> @@ -289,6 +289,8 @@ struct acpi_dep_data {
>  	acpi_handle supplier;
>  	acpi_handle consumer;
>  	bool honor_dep;
> +	bool met;
> +	bool free_when_met;
>  };
>  
>  /* Performance Management */
> Index: linux-pm/drivers/acpi/scan.c
> ===================================================================
> --- linux-pm.orig/drivers/acpi/scan.c
> +++ linux-pm/drivers/acpi/scan.c
> @@ -2029,8 +2029,6 @@ static u32 acpi_scan_check_dep(acpi_hand
>  	return count;
>  }
>  
> -static bool acpi_bus_scan_second_pass;
> -
>  static acpi_status acpi_bus_check_add(acpi_handle handle, bool check_dep,
>  				      struct acpi_device **adev_p)
>  {
> @@ -2050,10 +2048,8 @@ static acpi_status acpi_bus_check_add(ac
>  			return AE_OK;
>  
>  		/* Bail out if there are dependencies. */
> -		if (acpi_scan_check_dep(handle, check_dep) > 0) {
> -			acpi_bus_scan_second_pass = true;
> +		if (acpi_scan_check_dep(handle, check_dep) > 0)
>  			return AE_CTRL_DEPTH;
> -		}
>  
>  		fallthrough;
>  	case ACPI_TYPE_ANY:	/* for ACPI_ROOT_OBJECT */
> @@ -2301,6 +2297,12 @@ static bool acpi_scan_clear_dep_queue(st
>  	return true;
>  }
>  
> +static void acpi_scan_delete_dep_data(struct acpi_dep_data *dep)
> +{
> +	list_del(&dep->node);
> +	kfree(dep);
> +}
> +
>  static int acpi_scan_clear_dep(struct acpi_dep_data *dep, void *data)
>  {
>  	struct acpi_device *adev = acpi_get_acpi_dev(dep->consumer);
> @@ -2311,8 +2313,10 @@ static int acpi_scan_clear_dep(struct ac
>  			acpi_dev_put(adev);
>  	}
>  
> -	list_del(&dep->node);
> -	kfree(dep);
> +	if (dep->free_when_met)
> +		acpi_scan_delete_dep_data(dep);
> +	else
> +		dep->met = true;
>  
>  	return 0;
>  }
> @@ -2406,6 +2410,53 @@ struct acpi_device *acpi_dev_get_next_co
>  }
>  EXPORT_SYMBOL_GPL(acpi_dev_get_next_consumer_dev);
>  
> +static void acpi_scan_postponed_branch(acpi_handle handle)
> +{
> +	struct acpi_device *adev = NULL;
> +
> +	if (ACPI_FAILURE(acpi_bus_check_add(handle, false, &adev)))
> +		return;
> +
> +	acpi_walk_namespace(ACPI_TYPE_ANY, handle, ACPI_UINT32_MAX,
> +			    acpi_bus_check_add_2, NULL, NULL, (void **)&adev);
> +	acpi_bus_attach(adev, NULL);
> +}
> +
> +static void acpi_scan_postponed(void)
> +{
> +	struct acpi_dep_data *dep, *tmp;
> +
> +	mutex_lock(&acpi_dep_list_lock);
> +
> +	list_for_each_entry_safe(dep, tmp, &acpi_dep_list, node) {
> +		acpi_handle handle = dep->consumer;
> +
> +		/*
> +		 * Even though the lock is released here, tmp is guaranteed to
> +		 * be valid, because none of the list entries following dep is
> +		 * marked as "free when met" and so they cannot be deleted.
> +		 */
> +		mutex_unlock(&acpi_dep_list_lock);
> +
> +		/*
> +		 * In case there are multiple acpi_dep_list entries with the
> +		 * same consumer, skip the current entry if the consumer device
> +		 * object corresponding to it is present already.
> +		 */
> +		if (!acpi_fetch_acpi_dev(handle))
> +			acpi_scan_postponed_branch(handle);

acpi_fetch_acpi_dev(handle) does not need/take the acpi_dep_list_lock,
so you can avoid a needless unlock/lock in case acpi_fetch_acpi_dev(handle)
finds a device already, which will happen quite regular since devices
with _DEP lists regularly have more then 1 dep so they will be present
as consumer on the _DEP list more then once.

So maybe:

	list_for_each_entry_safe(dep, tmp, &acpi_dep_list, node) {
		acpi_handle handle = dep->consumer;
		struct acpi_device *device = acpi_fetch_acpi_dev(handle);

		/*
		 * In case there are multiple acpi_dep_list entries with the
		 * same consumer, skip scanning the current entry if the consumer
		 * device object corresponding to it is present already.
		 */
		if (device)
			goto check_dep;

		/*
		 * Even though the lock is released here, tmp is guaranteed to
		 * be valid, because none of the list entries following dep is
		 * marked as "free when met" and so they cannot be deleted.
		 */
		mutex_unlock(&acpi_dep_list_lock);
		acpi_scan_postponed_branch(handle);
		mutex_lock(&acpi_dep_list_lock);

check_dep:
		if (dep->met)
			acpi_scan_delete_dep_data(dep);
		else
			dep->free_when_met = true;
	}

?

Regards,

Hans



> +
> +	mutex_unlock(&acpi_dep_list_lock);
> +}
> +
>  /**
>   * acpi_bus_scan - Add ACPI device node objects in a given namespace scope.
>   * @handle: Root of the namespace scope to scan.
> @@ -2424,8 +2475,6 @@ int acpi_bus_scan(acpi_handle handle)
>  {
>  	struct acpi_device *device = NULL;
>  
> -	acpi_bus_scan_second_pass = false;
> -
>  	/* Pass 1: Avoid enumerating devices with missing dependencies. */
>  
>  	if (ACPI_SUCCESS(acpi_bus_check_add(handle, true, &device)))
> @@ -2438,19 +2487,9 @@ int acpi_bus_scan(acpi_handle handle)
>  
>  	acpi_bus_attach(device, (void *)true);
>  
> -	if (!acpi_bus_scan_second_pass)
> -		return 0;
> -
>  	/* Pass 2: Enumerate all of the remaining devices. */
>  
> -	device = NULL;
> -
> -	if (ACPI_SUCCESS(acpi_bus_check_add(handle, false, &device)))
> -		acpi_walk_namespace(ACPI_TYPE_ANY, handle, ACPI_UINT32_MAX,
> -				    acpi_bus_check_add_2, NULL, NULL,
> -				    (void **)&device);
> -
> -	acpi_bus_attach(device, NULL);
> +	acpi_scan_postponed();
>  
>  	return 0;
>  }
> 
> 
>