Re: [driver-core PATCH v10 3/9] driver core: Probe devices asynchronously instead of the driver

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, January 22, 2019 7:39:21 PM CET Alexander Duyck wrote:
> Probe devices asynchronously instead of the driver. This results in us
> seeing the same behavior if the device is registered before the driver or
> after. This way we can avoid serializing the initialization should the
> driver not be loaded until after the devices have already been added.
> 
> The motivation behind this is that if we have a set of devices that
> take a significant amount of time to load we can greatly reduce the time to
> load by processing them in parallel instead of one at a time. In addition,
> each device can exist on a different node so placing a single thread on one
> CPU to initialize all of the devices for a given driver can result in poor
> performance on a system with multiple nodes.
> 
> This approach can reduce the time needed to scan SCSI LUNs significantly.
> The only way to realize that speedup is by enabling more concurrency which
> is what is achieved with this patch.
> 
> To achieve this it was necessary to add a new member "async_driver" to the
> device_private structure to store the driver pointer while we wait on the
> deferred probe call.
> 
> Reviewed-by: Bart Van Assche <bvanassche@acm.org>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
>  drivers/base/base.h |    2 ++
>  drivers/base/bus.c  |   23 +++--------------------
>  drivers/base/dd.c   |   43 +++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 48 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/base/base.h b/drivers/base/base.h
> index b6966511b0ca..b405436ee28e 100644
> --- a/drivers/base/base.h
> +++ b/drivers/base/base.h
> @@ -65,6 +65,7 @@ struct driver_private {
>   *	binding of drivers which were unable to get all the resources needed by
>   *	the device; typically because it depends on another driver getting
>   *	probed first.
> + * @async_driver - pointer to device driver awaiting probe via async_probe
>   * @device - pointer back to the struct device that this structure is
>   * associated with.
>   * @dead - This device is currently either in the process of or has been
> @@ -80,6 +81,7 @@ struct device_private {
>  	struct klist_node knode_bus;
>  	struct klist_node knode_class;
>  	struct list_head deferred_probe;
> +	struct device_driver *async_driver;
>  	struct device *device;
>  	u8 dead:1;
>  };
> diff --git a/drivers/base/bus.c b/drivers/base/bus.c
> index cfa5b63b5582..0a58e969f8b7 100644
> --- a/drivers/base/bus.c
> +++ b/drivers/base/bus.c
> @@ -610,17 +610,6 @@ static ssize_t uevent_store(struct device_driver *drv, const char *buf,
>  }
>  static DRIVER_ATTR_WO(uevent);
>  
> -static void driver_attach_async(void *_drv, async_cookie_t cookie)
> -{
> -	struct device_driver *drv = _drv;
> -	int ret;
> -
> -	ret = driver_attach(drv);
> -
> -	pr_debug("bus: '%s': driver %s async attach completed: %d\n",
> -		 drv->bus->name, drv->name, ret);
> -}
> -
>  /**
>   * bus_add_driver - Add a driver to the bus.
>   * @drv: driver.
> @@ -653,15 +642,9 @@ int bus_add_driver(struct device_driver *drv)
>  
>  	klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers);
>  	if (drv->bus->p->drivers_autoprobe) {
> -		if (driver_allows_async_probing(drv)) {
> -			pr_debug("bus: '%s': probing driver %s asynchronously\n",
> -				drv->bus->name, drv->name);
> -			async_schedule(driver_attach_async, drv);
> -		} else {
> -			error = driver_attach(drv);
> -			if (error)
> -				goto out_unregister;
> -		}
> +		error = driver_attach(drv);
> +		if (error)
> +			goto out_unregister;
>  	}
>  	module_add_driver(drv->owner, drv);
>  
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 34556359c7da..627ad05064e0 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -925,6 +925,30 @@ int device_driver_attach(struct device_driver *drv, struct device *dev)
>  	return ret;
>  }
>  
> +static void __driver_attach_async_helper(void *_dev, async_cookie_t cookie)
> +{
> +	struct device *dev = _dev;
> +	struct device_driver *drv;
> +	int ret = 0;
> +
> +	__device_driver_lock(dev, dev->parent);
> +
> +	drv = dev->p->async_driver;
> +
> +	/*
> +	 * If device has been removed or someone has already successfully
> +	 * bound a driver before us just skip the driver probe call.
> +	 */
> +	if (!dev->p->dead && !dev->driver)
> +		ret = driver_probe_device(drv, dev);
> +
> +	__device_driver_unlock(dev, dev->parent);
> +
> +	dev_dbg(dev, "driver %s async attach completed: %d\n", drv->name, ret);
> +
> +	put_device(dev);
> +}
> +
>  static int __driver_attach(struct device *dev, void *data)
>  {
>  	struct device_driver *drv = data;
> @@ -952,6 +976,25 @@ static int __driver_attach(struct device *dev, void *data)
>  		return ret;
>  	} /* ret > 0 means positive match */
>  
> +	if (driver_allows_async_probing(drv)) {
> +		/*
> +		 * Instead of probing the device synchronously we will
> +		 * probe it asynchronously to allow for more parallelism.
> +		 *
> +		 * We only take the device lock here in order to guarantee
> +		 * that the dev->driver and async_driver fields are protected
> +		 */
> +		dev_dbg(dev, "probing driver %s asynchronously\n", drv->name);
> +		device_lock(dev);
> +		if (!dev->driver) {
> +			get_device(dev);
> +			dev->p->async_driver = drv;
> +			async_schedule(__driver_attach_async_helper, dev);
> +		}
> +		device_unlock(dev);
> +		return 0;
> +	}
> +
>  	device_driver_attach(drv, dev);
>  
>  	return 0;
> 
>