On Tue, 8 Mar 2016, David Mosberger-Tang wrote:
> From: David Mosberger <[email protected]>
>
> usb_submit_urb() may take quite long to execute. For example, a
> single sg list may have 30 or more entries, possibly leading to that
> many calls to DMA-map pages. This can cause interrupt latency of
> several hundred micro-seconds.
>
> Avoid the problem by releasing the io->lock spinlock and re-enabling
> interrupts before calling usb_submit_urb(). This opens races with
> usb_sg_cancel() and sg_complete(). Handle those races by using
> usb_block_urb() to stop URBs from being submitted after
> usb_sg_cancel() or sg_complete() with error.
>
> Note that usb_unlink_urb() is guaranteed to return -ENODEV if
> !io->urbs[i]->dev and since the -ENODEV case is already handled,
> we don't have to check for !io->urbs[i]->dev explicitly.
>
> Before this change, reading 512MB from an ext3 filesystem on a USB
> memory stick showed a throughput of 12 MB/s with about 500 missed
> deadlines.
>
> With this change, reading the same file gave the same throughput but
> only one or two missed deadlines.
>
> Signed-off-by: David Mosberger <[email protected]>
Pretty good. Only one change is really needed.
> @@ -515,12 +516,10 @@ void usb_sg_wait(struct usb_sg_request *io)
> int retval;
>
> io->urbs[i]->dev = io->dev;
> + spin_unlock_irq(&io->lock);
> +
> retval = usb_submit_urb(io->urbs[i], GFP_ATOMIC);
This should now be GFP_NOIO.
> - /* after we submit, let completions or cancellations fire;
> - * we handshake using io->status.
> - */
> - spin_unlock_irq(&io->lock);
> switch (retval) {
> /* maybe we retrying will recover */
> case -ENXIO: /* hc didn't queue this one */
> @@ -590,8 +589,8 @@ void usb_sg_cancel(struct usb_sg_request *io)
> for (i = 0; i < io->entries; i++) {
> int retval;
>
> - if (!io->urbs[i]->dev)
> - continue;
> + usb_block_urb(io->urbs[i]);
> +
> retval = usb_unlink_urb(io->urbs[i]);
> if (retval != -EINPROGRESS
> && retval != -ENODEV
Strictly speaking, this loop should run backward. Then the
spin_unlock() could be replaced with spin_unlock_irqrestore().
In fact, the whole routine could be restructured like this:
spin_lock_irqsave(&io->lock, flags);
/* shut everything down, if it isn't already */
if (io->status) {
spin_unlock_irqrestore(&io->lock, flags);
return;
}
io->status = -ECONNRESET;
spin_unlock_irqrestore(&io->lock, flags);
for (i = io->entries - 1; i >= 0; --i) {
...
But that should be done in a separate patch. It's not critical anyway;
cancelling I/O is relatively rare and unimportant.
Alan Stern
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