[PATCH] reduced latency in dentry and inode cache shrinking

Shrinking a huge number of dentries or inodes can hold dcache_lock or
inode_lock for a long time.  Not only does this hold off preemption -
holding those locks basically shuts down the whole VFS.

A neat fix for all such caches is to chunk the work up at the
shrink_slab() level.

I made the chunksize pretty small, for scalability reasons - avoid
holding the lock for too long so another CPU can come in, acquire it
and go off to do some work.

mm/vmscan.c

@@ -155,10 +155,18 @@ static int shrink_slab(long scanned,  unsigned int gfp_mask)
 		do_div(delta, pages + 1);
 		shrinker->nr += delta;
 		if (shrinker->nr > SHRINK_BATCH) {
-			long nr = shrinker->nr;
+			long nr_to_scan = shrinker->nr;
 
 			shrinker->nr = 0;
-			(*shrinker->shrinker)(nr, gfp_mask);
+			while (nr_to_scan) {
+				long this_scan = nr_to_scan;
+
+				if (this_scan > 128)
+					this_scan = 128;
+				(*shrinker->shrinker)(this_scan, gfp_mask);
+				nr_to_scan -= this_scan;
+				cond_resched();
+			}
 		}
 	}
 	up(&shrinker_sem);