perf callchain: Spare double comparison of callchain first entry

When a new callchain child branch matches an existing one in the rbtree,
the comparison of its first entry is performed twice:

1) From append_chain_children() on branch lookup

2) If 1) reports a match, append_chain() then compares all entries of
the new branch against the matching node in the rbtree, and this
comparison includes the first entry of the new branch again.

Lets shortcut this by performing the whole comparison only from
append_chain() which then returns the result of the comparison between
the first entry of the new branch and the iterating node in the rbtree.
If the first entry matches, the lookup on the current level of siblings
stops and propagates to the children of the matching nodes.

This results in less comparisons performed by the CPU.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1389713836-13375-3-git-send-email-fweisbec@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/callchain.c

@@ -15,6 +15,8 @@
 #include <errno.h>
 #include <math.h>
 
+#include "asm/bug.h"
+
 #include "hist.h"
 #include "util.h"
 #include "sort.h"
@@ -358,19 +360,14 @@ append_chain_children(struct callchain_node *root,
 	/* lookup in childrens */
 	while (*p) {
 		s64 ret;
-		struct callchain_list *cnode;
 
 		parent = *p;
 		rnode = rb_entry(parent, struct callchain_node, rb_node_in);
-		cnode = list_first_entry(&rnode->val, struct callchain_list,
-					 list);
 
-		/* just check first entry */
-		ret = match_chain(node, cnode);
-		if (ret == 0) {
-			append_chain(rnode, cursor, period);
+		/* If at least first entry matches, rely to children */
+		ret = append_chain(rnode, cursor, period);
+		if (ret == 0)
 			goto inc_children_hit;
-		}
 
 		if (ret < 0)
 			p = &parent->rb_left;
@@ -396,6 +393,7 @@ append_chain(struct callchain_node *root,
 	u64 start = cursor->pos;
 	bool found = false;
 	u64 matches;
+	int cmp = 0;
 
 	/*
 	 * Lookup in the current node
@@ -410,7 +408,8 @@ append_chain(struct callchain_node *root,
 		if (!node)
 			break;
 
-		if (match_chain(node, cnode) != 0)
+		cmp = match_chain(node, cnode);
+		if (cmp)
 			break;
 
 		found = true;
@@ -420,9 +419,10 @@ append_chain(struct callchain_node *root,
 
 	/* matches not, relay no the parent */
 	if (!found) {
+		WARN_ONCE(!cmp, "Chain comparison error\n");
 		cursor->curr = curr_snap;
 		cursor->pos = start;
-		return -1;
+		return cmp;
 	}
 
 	matches = cursor->pos - start;