[POWERPC] hvc_console: Polling mode timer backoff

Add a back-off mechanism to hvc_console's polling logic.   This change
drops the timers/second ratio from ~90 to ~1/2 while the console is
idle.
This change is most noticeable when watching /proc/timer_stats output.

This only affects when the hvc_console is running in poll mode, i.e.
power4 and cell systems.

I've tested on Power4, Michael Ellerman has both contributed to the
patch and tested on cell.

Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 25 insertions, 6 deletions

diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index a0a88aa23f5b..fc9bc7709fd9 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -47,8 +47,6 @@
 #define HVC_MAJOR       229
 #define HVC_MINOR       0
 
-#define TIMEOUT         (10)
-
 /*
  * Wait this long per iteration while trying to push buffered data to the
  * hypervisor before allowing the tty to complete a close operation.
@@ -550,6 +548,18 @@ static int hvc_chars_in_buffer(struct tty_struct *tty)
         return hp->n_outbuf;
 }
 
+/*
+ * timeout will vary between the MIN and MAX values defined here.  By default
+ * and during console activity we will use a default MIN_TIMEOUT of 10.  When
+ * the console is idle, we increase the timeout value on each pass through
+ * msleep until we reach the max.  This may be noticeable as a brief (average
+ * one second) delay on the console before the console responds to input when
+ * there has been no input for some time.
+ */
+#define MIN_TIMEOUT             (10)
+#define MAX_TIMEOUT             (2000)
+static u32 timeout = MIN_TIMEOUT;
+
 #define HVC_POLL_READ   0x00000001
 #define HVC_POLL_WRITE  0x00000002
 
@@ -642,9 +652,14 @@ static int hvc_poll(struct hvc_struct *hp)
  bail:
         spin_unlock_irqrestore(&hp->lock, flags);
 
-        if (read_total)
+        if (read_total) {
+                /* Activity is occurring, so reset the polling backoff value to
+                   a minimum for performance. */
+                timeout = MIN_TIMEOUT;
+
                 tty_flip_buffer_push(tty);
-        
+        }
+
         return poll_mask;
 }
 
@@ -688,8 +703,12 @@ int khvcd(void *unused)
                 if (!hvc_kicked) {
                         if (poll_mask == 0)
                                 schedule();
-                        else
-                                msleep_interruptible(TIMEOUT);
+                        else {
+                                if (timeout < MAX_TIMEOUT)
+                                        timeout += (timeout >> 6) + 1;
+
+                                msleep_interruptible(timeout);
+                        }
                 }
                 __set_current_state(TASK_RUNNING);
         } while (!kthread_should_stop());