[PATCH 6.6 0667/1266] perf sched: Fix idle-hist callchain display using wrong rb_first variant

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

6.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Arnaldo Carvalho de Melo <acme@redhat.com>

[ Upstream commit d9b99dc8148e0c1f5da3942131b47e0d21187a32 ]

timehist_print_idlehist_callchain() calls rb_first_cached() on
sorted_root, but the sort function (callchain_param.sort) populates it
via rb_insert_color() on the plain rb_root member — not the cached
variant.  This means rb_leftmost is never set, so rb_first_cached()
always returns NULL and the entire callchain summary is silently
dropped from --idle-hist output.

The original code in ba957ebb54893aca ("perf sched timehist: Show
callchains for idle stat") was correct — it used struct rb_root and
rb_first().  The bug was introduced when sorted_root was converted to
rb_root_cached without converting the sort insertion path to use
rb_insert_color_cached().

Use rb_first(&root->rb_root) to match how the tree was populated.

Fixes: cb4c13a5137766c3 ("perf sched: Use cached rbtrees")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 tools/perf/builtin-sched.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 8b1573515c5781..db4ceebab8d8ad 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -2906,7 +2906,8 @@ static size_t timehist_print_idlehist_callchain(struct rb_root_cached *root)
 	size_t ret = 0;
 	FILE *fp = stdout;
 	struct callchain_node *chain;
-	struct rb_node *rb_node = rb_first_cached(root);
+	/* sort() uses rb_insert_color() on rb_root, not rb_root_cached */
+	struct rb_node *rb_node = rb_first(&root->rb_root);
 
 	printf("  %16s  %8s  %s\n", "Idle time (msec)", "Count", "Callchains");
 	printf("  %.16s  %.8s  %.50s\n", graph_dotted_line, graph_dotted_line,
-- 
2.53.0