[PATCH] Add mpage_writepages() scheduling point

From: Jens Axboe <axboe@suse.de>

Takashi did some nice latency testing of the current kernel (with -mm
writeback changes), and the biggest offender in general core is
mpage_writepages().

fs/mpage.c

@@ -679,6 +679,7 @@ mpage_writepages(struct address_space *mapping,
 			}
 		}
 		pagevec_release(&pvec);
+		cond_resched();
 	}
 	if (!scanned && !done) {
 		/*