[PATCH] Reduce context switch rate due to the random driver

add_disk_randomness() is causing a context switch per disk request.  It
is scheduling process-context work one timer tick in the future for
every request.

But is has a buffer for this, so change it to not set up the
process-context work until that buffer is half full.

The patch reduces the context switch rate during a 20 megabyte/sec
write to scsi from 150/sec to 50/sec.

drivers/char/random.c

@@ -660,14 +660,16 @@ void batch_entropy_store(u32 a, u32 b, int num)
 	batch_entropy_credit[batch_head] = num;
 
 	new = (batch_head+1) & (batch_max-1);
-	if (new != batch_tail) {
+	if ((unsigned)(new - batch_tail) >= (unsigned)(batch_max / 2)) {
 		/*
 		 * Schedule it for the next timer tick:
 		 */
 		schedule_delayed_work(&batch_work, 1);
 		batch_head = new;
-	} else {
+	} else if (new == batch_tail) {
 		DEBUG_ENT("batch entropy buffer full\n");
+	} else {
+		batch_head = new;
 	}
 }