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

[Dan Williams]

On Thu, Sep 27, 2018 at 8:31 AM Alexander Duyck
<alexander.h.duyck@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 9/26/2018 5:48 PM, Dan Williams wrote:
> > 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.
>
> The thing is the current async behavior already does this if the driver
> is loaded before the device is added. All I am doing is making the
> behavior with the driver loaded first the standard instead of letting it
> work the other way around. This way we get consistent behavior.

Ok, I can see the consistency argument. It's still a behavior change
that needs testing. Configurations that have been living with the
default of synchronous probing of the devices on the bus for a later
arriving driver might be surprised.

That said, I was confusing async probing with device registration in
my thinking, so some of the discovery order / naming concerns may not
be as bad as I was imagining. Sub-systems that would be broken by this
behavior change would already be broken if a driver is built-in vs
module.

So, consider this, an Acked-by.

> > 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.
>
> So where I actually saw this was with the pmem legacy setup I had. After
> doing all the work to parallelize things in the driver it had no effect.
> That was because the nd_pmem driver wasn't loaded yet so all the
> device_add calls did is add the device but didn't attach the nd_pmem
> driver. Then when the driver loaded it serialized the probe calls
> resulting in it taking twice as long as it needed to in order to
> initialize the memory.
>
> This seems to affect standard persistent memory as well. The only
> difference is that instead of probing the device on the first pass we
> kick it back and reprobe it in nd_pmem_probe/nd_pfn_probe in order to
> set the correct personality and that in turn allows us to asynchronously
> reschedule the work on the correct CPU and deserialize it.

I don't see any problems with this series with the nvdimm unit tests,
but note those tests do not check for discovery order / naming
discrepancies.