Re: [patch 5/3] fastboot: sync the async execution before late_initcall and move level 6s (sync) first

On Thu, 31 Jul 2008, Simon Arlott wrote:

If it were a module then it would block in a separate thread and
wouldn't hold up the main init process.

Right, but I want to compile all of this into the kernel.

Okay, that explains things. But this means that changes which are
helpful on your system might not be so helpful (or might even be
detrimental) on other people's systems.

It's not entirely clear why usblp is blocking at all. Probably because
it is waiting on the device semaphores for devices that are currently
being probed -- the driver core won't allow two threads to probe the
same device concurrently.

Ok - so there could be some big improvements to be had by making the hcd
init happen as early as possible and the device initcalls later?

Maybe. Perhaps a better approach would be to make the device driver
initcalls before there are any devices for their probe routines to
block on.

Driver probing is always going to be a bottleneck, even for
non-matching device/driver pairs. That's because the driver core
acquires the device semaphore even before calling the bus's match
routine. When a new driver is registered, the driver core just goes
down the list of all devices registered on the bus, locking each one in
turn and trying to match & probe it.

So could HCD init be moved higher in the initcall order so that the devices
have been initialised enough by khubd that device driver initcalls take
much less time?

I'm not really sure how to do that, except by doing usb/core/ usb/host/
and then usb/class/ usb/storage/ etc....

Try doing this instead: Put a 5-second delay at the start of
hub_thread(). That will give the in-kernel device drivers time to
initialize before any USB devices -- other than root hubs -- are
discovered. That should speed up the init timings.

For everyone else, it will slow down USB device discovery by 5 seconds.
I don't know how important that is, considering how much other stuff is
going on at boot time. You may find that 5 seconds is more than you
need; perhaps 1 second will be enough.

Alan Stern

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