Re: [driver-core PATCH v3 3/5] driver core: Probe devices asynchronously instead of the driver

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Oct 12, 2018 at 12:10 AM Alexander Duyck
<alexander.h.duyck@linux.intel.com> wrote:
>
> This change makes it so that we 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.
>
> One issue I can see with this patch is that I am using the
> dev_set/get_drvdata functions to store the driver in the device while I am
> waiting on the asynchronous init to complete. For now I am protecting it by
> using the lack of a dev->driver and the device lock.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  drivers/base/bus.c |   23 +++--------------------
>  drivers/base/dd.c  |   42 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/base/bus.c b/drivers/base/bus.c
> index 8bfd27ec73d6..2a17bed657ec 100644
> --- a/drivers/base/bus.c
> +++ b/drivers/base/bus.c
> @@ -616,17 +616,6 @@ 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.
> @@ -659,15 +648,9 @@ 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 169412ee4ae8..5ba366c1cb83 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -864,6 +864,29 @@ void device_initial_probe(struct device *dev)
>         __device_attach(dev, true);
>  }
>
> +static void __driver_attach_async_helper(void *_dev, async_cookie_t cookie)
> +{
> +       struct device *dev = _dev;
> +
> +       if (dev->parent && dev->bus->need_parent_lock)
> +               device_lock(dev->parent);
> +       device_lock(dev);
> +
> +       if (!dev->driver) {
> +               struct device_driver *drv = dev_get_drvdata(dev);
> +
> +               driver_probe_device(drv, dev);
> +       }
> +
> +       dev_dbg(dev, "async probe completed\n");
> +
> +       device_unlock(dev);
> +       if (dev->parent && dev->bus->need_parent_lock)
> +               device_unlock(dev->parent);

The above duplicates some code from __driver_attach().

You could avoid that by adding one more function to call directly from
__driver_attach() in the "sync" case and from here, along the lines of
what the PM code does.

> +
> +       put_device(dev);
> +}
> +
>  static int __driver_attach(struct device *dev, void *data)
>  {
>         struct device_driver *drv = data;
> @@ -891,6 +914,25 @@ 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 driver_data fields are protected
> +                */
> +               dev_dbg(dev, "scheduling asynchronous probe\n");
> +               device_lock(dev);
> +               if (!dev->driver) {
> +                       get_device(dev);
> +                       dev_set_drvdata(dev, drv);
> +                       async_schedule(__driver_attach_async_helper, dev);
> +               }
> +               device_unlock(dev);
> +               return 0;
> +       }
> +
>         if (dev->parent && dev->bus->need_parent_lock)
>                 device_lock(dev->parent);
>         device_lock(dev);
>