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

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Date:	Thu, 31 Mar 2016 01:00:10 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Andres Freund <andres@...razel.de>
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 Andres,

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!

> 
> It's somewhat weird that --stdio doesn't show the problem, but --tui
> does. Hm.
> 
> 
> 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...

> 
> 
> I think that actually explains why stdio works - nodes are always
> unfolded in it, thus ->has_children isn't looked at.

Right, the ->has_children thing is only for TUI code which
folds/collapses the entries dynamically.

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

Thanks,
Namhyung