USB: EHCI: fix performance regression

This patch (as1099) fixes a performance regression in ehci-hcd.  The
fundamental problem is that queue headers get removed from the
schedule too quickly, since the code checks for a counter advancing
rather than making an actual time-based check.  The latency involved
in removing the queue header and then relinking it can severely
degrade certain kinds of workloads.

The patch replaces a simple counter with a timestamp derived from the
controller's uframe value.  In addition, the delay for unlinking an
idle queue header is increased from 5 ms to 10 ms; since some
controllers (nVidia) have a latency of up to 1 ms for unlinking, this
reduces the relative impact from 20% to 10%.

Finally, a logical error left over from the IAA watchdog-timer
conversion is corrected.  Now the driver will always either unlink an
idle queue header or set up a timer to unlink it later.  The old code
would sometimes fail to do either.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: David Brownell <david-b@pacbell.net>
Cc: Leonid <leonidv11@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 10 insertions, 8 deletions

diff --git a/drivers/usb/host/ehci-hcd.c b/drivers/usb/host/ehci-hcd.c
index 369a8a5ea7bb..3e3c5d3ea0ad 100644
--- a/drivers/usb/host/ehci-hcd.c
+++ b/drivers/usb/host/ehci-hcd.c
@@ -84,7 +84,8 @@ static const char	hcd_name [] = "ehci_hcd";
 #define EHCI_IAA_MSECS          10              /* arbitrary */
 #define EHCI_IO_JIFFIES         (HZ/10)         /* io watchdog > irq_thresh */
 #define EHCI_ASYNC_JIFFIES      (HZ/20)         /* async idle timeout */
-#define EHCI_SHRINK_JIFFIES     (HZ/200)        /* async qh unlink delay */
+#define EHCI_SHRINK_JIFFIES     (HZ/100)        /* async qh unlink delay */
+#define EHCI_SHRINK_UFRAMES     (10*8)          /* same value in uframes */
 
 /* Initial IRQ latency:  faster than hw default */
 static int log2_irq_thresh = 0;         // 0 to 6
diff --git a/drivers/usb/host/ehci-q.c b/drivers/usb/host/ehci-q.c
index b85b54160cda..5200481deb27 100644
--- a/drivers/usb/host/ehci-q.c
+++ b/drivers/usb/host/ehci-q.c
@@ -1116,8 +1116,7 @@ static void scan_async (struct ehci_hcd *ehci)
         struct ehci_qh          *qh;
         enum ehci_timer_action  action = TIMER_IO_WATCHDOG;
 
-        if (!++(ehci->stamp))
-                ehci->stamp++;
+        ehci->stamp = ehci_readl(ehci, &ehci->regs->frame_index);
         timer_action_done (ehci, TIMER_ASYNC_SHRINK);
 rescan:
         qh = ehci->async->qh_next.qh;
@@ -1148,12 +1147,14 @@ rescan:
                          * doesn't stay idle for long.
                          * (plus, avoids some kind of re-activation race.)
                          */
-                        if (list_empty (&qh->qtd_list)) {
-                                if (qh->stamp == ehci->stamp)
+                        if (list_empty(&qh->qtd_list) &&
+                                        qh->qh_state == QH_STATE_LINKED) {
+                                if (!ehci->reclaim &&
+                                    ((ehci->stamp - qh->stamp) & 8191) >=
+                                                EHCI_SHRINK_UFRAMES)
+                                        start_unlink_async(ehci, qh);
+                                else
                                         action = TIMER_ASYNC_SHRINK;
-                                else if (!ehci->reclaim
-                                            && qh->qh_state == QH_STATE_LINKED)
-                                        start_unlink_async (ehci, qh);
                         }
 
                         qh = qh->qh_next.qh;