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

[Dan Williams]

On Wed, Sep 26, 2018 at 2:51 PM Alexander Duyck
<alexander.h.duyck@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
[..]
> @@ -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);

I'm not sure async drivers / sub-systems are ready for their devices
to show up in parallel. While userspace should not be relying on
kernel device names, people get upset when devices change kernel names
from one boot to the next, and I can see this change leading to that
scenario.

If a driver / sub-system wants more parallelism than what
driver_allows_async_probing() provides it should do it locally, for
example, like libata does.