linux-kernel - Re: "perf hists browser: Support flat callchains" appears to have broken parent reporting

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Message-ID: <20160330190245.GB13305@awork2.anarazel.de>
Date:	Wed, 30 Mar 2016 21:02:45 +0200
From:	Andres Freund <andres@...razel.de>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: "perf hists browser: Support flat callchains" appears to have
 broken parent reporting

Hi!

On 2016-03-31 01:00:10 +0900, Namhyung Kim wrote:
> On Wed, Mar 30, 2016 at 04:19:26PM +0200, Andres Freund wrote:
> > On 2016-03-30 10:46:34 -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Wed, Mar 30, 2016 at 02:34:18PM +0200, Andres Freund escreveu:
> > > > Hi,
> > > > 
> > > > 4b3a3212233a - "perf hists browser: Support flat callchains" seems to
> > > > have broken callchain display in tui mode when using !flat mode, or at
> > > > least changed it in an unintended manner.
> > > 
> > > humm, at first I thought this would be related to --percent-limit...
> > 
> > I'm not using --percent-limit. Just to be sure, I did explicitly set it
> > to various values, and it looks unrelated.
> > 
> > > What tree/branch are you using? Can you try pressing 'L' to play with
> > > the percent limit?
> > 
> > I'm primarily using linus' tree, and bisected the behavioural down to
> > that individual commit.
> 
> Thanks for reporting and finding this!

No problem.  I'm somewhat surprised to be the first to report this, the
behavioural change confused me quite a bit.  Maybe it took others about
as long as me to figure out it's actually a perf report problem ;)

> > I don't know the perf code at all, but skimming through the commit, the
> > following hunk looks suspicious:
> > 
> > @@ -263,7 +295,7 @@ static void callchain_node__init_have_children(struct callchain_node *node,
> >     chain = list_entry(node->val.next, struct callchain_list, list);
> >     chain->has_children = has_sibling;
> >  
> > -   if (!list_empty(&node->val)) {
> > +   if (node->val.next != node->val.prev) {
> >         chain = list_entry(node->val.prev, struct callchain_list, list);
> >         chain->has_children = !RB_EMPTY_ROOT(&node->rb_root);
> >     }
> > 
> > Reverting that individual change fixes things.  I'm not actually sure
> > what the post 4b3a3212233a version actually tests for?
> 
> Yeah, this is it.  It's my fault that I thought if the first chain
> (node->val.next) was set by has_sibling, no need to go to the body
> of the "if" statement when next == prev case.  But it's not...

> Do you mind resending the fix as a formal patch with my ack ?

Done.

I couldn't really explain the problem, so I handwaved the actual problem
away: I've not really grasped how the rbtree, callchain_lists and
callchain_node are intertwined.

My symptom description probably is inaccurate as well.


>From 017d80e3c98c7237183ac20bb1cfc81bd5e9a474 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@...razel.de>
Date: Wed, 30 Mar 2016 19:39:28 +0200
Subject: [PATCH] perf hists: Fix determination of a callchain node's
 childlessness.

4b3a3212233a ("perf hists browser: Support flat callchains")
over-aggressively tried to optimize
callchain_node__init_have_children().

That lead to --tui mode not allowing to expand call chain elements if a
call chain element had only one parent. That's why --inverted callgraphs
looked halfway sane, but plain ones didn't.

Revert that individual optimization, it wasn't really related to the
rest of the commit.

Fixes: 4b3a3212233a042f48b7b8fedc64933e1ccd8643

Acked-by: Namhyung Kim <namhyung@...nel.org>
Signed-off-by: Andres Freund <andres@...razel.de>
---
 tools/perf/ui/browsers/hists.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/ui/browsers/hists.c b/tools/perf/ui/browsers/hists.c
index 4b98165..2a83414 100644
--- a/tools/perf/ui/browsers/hists.c
+++ b/tools/perf/ui/browsers/hists.c
@@ -337,7 +337,7 @@ static void callchain_node__init_have_children(struct callchain_node *node,
 	chain = list_entry(node->val.next, struct callchain_list, list);
 	chain->has_children = has_sibling;
 
-	if (node->val.next != node->val.prev) {
+	if (!list_empty(&node->val)) {
 		chain = list_entry(node->val.prev, struct callchain_list, list);
 		chain->has_children = !RB_EMPTY_ROOT(&node->rb_root);
 	}
-- 
2.7.0.229.g701fa7f.dirty