xenpaging: improve performance in policy_choose_victim

policy_choose_victim() is one of the bottlenecks in xenpaging. It is called
alot to find free bits in the fragmented bitmaps.

Reduce turnaround time by skipping longs with all bits set.
Adjust wrap detection in loop.

v2:
 - fix copy&paste error, bitmap was tested twice

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson.citrix.com>

tools/xenpaging/policy_default.c

@@ -80,33 +80,58 @@ int policy_init(struct xenpaging *paging)
 unsigned long policy_choose_victim(struct xenpaging *paging)
 {
     xc_interface *xch = paging->xc_handle;
-    unsigned long wrap = current_gfn;
+    unsigned long i;
 
-    do
+    /* One iteration over all possible gfns */
+    for ( i = 0; i < max_pages; i++ )
     {
+        /* Try next gfn */
         current_gfn++;
+
+        /* Restart on wrap */
         if ( current_gfn >= max_pages )
             current_gfn = 0;
-        /* Could not nominate any gfn */
-        if ( wrap == current_gfn )
+
+        if ( (current_gfn & (BITS_PER_LONG - 1)) == 0 )
         {
-            paging->use_poll_timeout = 1;
-            /* Count wrap arounds */
-            unconsumed_cleared++;
-            /* Force retry every few seconds (depends on poll() timeout) */
-            if ( unconsumed_cleared > 123)
+            /* All gfns busy */
+            if ( ~bitmap[current_gfn >> ORDER_LONG] == 0 || ~unconsumed[current_gfn >> ORDER_LONG] == 0 )
             {
-                /* Force retry of unconsumed gfns */
-                bitmap_clear(unconsumed, max_pages);
-                unconsumed_cleared = 0;
-                DPRINTF("clearing unconsumed, wrap %lx", wrap);
-                /* One more round before returning ENOSPC */
+                current_gfn += BITS_PER_LONG;
+                i += BITS_PER_LONG;
                 continue;
             }
-            return INVALID_MFN;
         }
+
+        /* gfn busy */
+        if ( test_bit(current_gfn, bitmap) )
+            continue;
+
+        /* gfn already tested */
+        if ( test_bit(current_gfn, unconsumed) )
+            continue;
+
+        /* gfn found */
+        break;
+    }
+
+    /* Could not nominate any gfn */
+    if ( i >= max_pages )
+    {
+        /* No more pages, wait in poll */
+        paging->use_poll_timeout = 1;
+        /* Count wrap arounds */
+        unconsumed_cleared++;
+        /* Force retry every few seconds (depends on poll() timeout) */
+        if ( unconsumed_cleared > 123)
+        {
+            /* Force retry of unconsumed gfns on next call */
+            bitmap_clear(unconsumed, max_pages);
+            unconsumed_cleared = 0;
+            DPRINTF("clearing unconsumed, current_gfn %lx", current_gfn);
+        }
+        return INVALID_MFN;
     }
-    while ( test_bit(current_gfn, bitmap) || test_bit(current_gfn, unconsumed) );
 
     set_bit(current_gfn, unconsumed);
     return current_gfn;