[RFC PATCH 0/4] USB: HCD/EHCI: giveback of URB in tasklet context

[Alan Stern]

On Thu, 13 Jun 2013, Ming Lei wrote:

> - using interrupt threaded handler(default)
>         33.440 MB/sec
> 
> - using tasklet(#undef USB_HCD_THREADED_IRQ)
>         34.29 MB/sec
> 
> - using hard interrupt handler(by removing HCD_BH in ehci-hcd.c )
>         34.260 MB/s
> 
> 
> So looks usb mass storage performance loss can be observed with
> interrupt threaded handler because one mass storage read/write sectors
> requires at least 3 interrupts which wake up usb-storage thread 3 times
> (each interrupt wakeup the usb-storage each time), introducing irq threaded
> handler will make 2 threads to be waken up about 6 times for one read/write.
> 
> I think usb mass storage transfer handler need to be rewritten, otherwise
> it may become worsen after using irq threaded handler in USB 3.0.(the
> above device can reach >120MB/sec with hardware handler or tasklet handler,
> which means about ~3K interrupts/sec, so ~6K contexts switch in case of
> using irq threaded handler)
> 
> So how about supporting tasklet first, then convert to interrupt
> threaded handler
> after usb mass storage transfer is rewritten without performance loss?
> (rewriting
> usb mass storage transfer handler may need some time and work since storage
> stability/correctness is extremely important, :-)

Maybe we should simply copy what the networking people do.  They are 
very concerned about performance and latency; whatever technique they 
use should be good for USB too.

> Also another problem with irq threaded handler is that there is no sort of
> tasklet_schedule() interface to wakeup the thread handler manually, so
> I have to use work to schedule some URB giveback from drivers(root hub
> transfer, unlink),  even though that isn't a big deal but will cause code a bit
> much/complicated, :-)

Yes, I was going to bring that up.

Thomas, sometimes we need the IRQ handler thread to do some work even
though an interrupt hasn't occurred.  Is there an API for this, or can 
one be added?

Alan Stern