From: David Mosberger <davidm@xxxxxxxxxx>
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 <davidm@xxxxxxxxxx>
---
drivers/usb/core/message.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)
diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 8e641b5..e13a2e5 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -302,9 +302,10 @@ static void sg_complete(struct urb *urb)
*/
spin_unlock(&io->lock);
for (i = 0, found = 0; i < io->entries; i++) {
- if (!io->urbs[i] || !io->urbs[i]->dev)
+ if (!io->urbs[i])
continue;
if (found) {
+ usb_block_urb(io->urbs[i]);
retval = usb_unlink_urb(io->urbs[i]);
if (retval != -EINPROGRESS &&
retval != -ENODEV &&
@@ -515,12 +516,10 @@ void usb_sg_wait(struct usb_sg_request *io)
int retval;
io->urbs[i]->dev = io->dev;
- retval = usb_submit_urb(io->urbs[i], GFP_ATOMIC);
-
- /* after we submit, let completions or cancellations fire;
- * we handshake using io->status.
- */
spin_unlock_irq(&io->lock);
+
+ retval = usb_submit_urb(io->urbs[i], GFP_NOIO);
+
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
--
1.9.1
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