linux-kernel - Re: [PATCH 34/37] perf hists browser: Support flat callchains

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Message-ID: <20151124144551.GB30441@redhat.com>
Date:	Tue, 24 Nov 2015 12:45:51 -0200
From:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Kan Liang <kan.liang@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 34/37] perf hists browser: Support flat callchains

Em Tue, Nov 24, 2015 at 02:27:08PM +0900, Namhyung Kim escreveu:
> On Mon, Nov 23, 2015 at 04:16:48PM +0100, Frederic Weisbecker wrote:
> > On Thu, Nov 19, 2015 at 02:53:20PM -0300, Arnaldo Carvalho de Melo wrote:
> > > From: Namhyung Kim <namhyung@...nel.org>
> > [...]
> > 
> > > +int callchain_node__make_parent_list(struct callchain_node *node)
> > > +{
> > > +	struct callchain_node *parent = node->parent;
> > > +	struct callchain_list *chain, *new;
> > > +	LIST_HEAD(head);
> > > +
> > > +	while (parent) {
> > > +		list_for_each_entry_reverse(chain, &parent->val, list) {
> > > +			new = malloc(sizeof(*new));
> > > +			if (new == NULL)
> > > +				goto out;
> > > +			*new = *chain;
> > > +			new->has_children = false;
> > > +			list_add_tail(&new->list, &head);
> > > +		}
> > > +		parent = parent->parent;
> > > +	}
> > > +
> > > +	list_for_each_entry_safe_reverse(chain, new, &head, list)
> > > +		list_move_tail(&chain->list, &node->parent_val);
> > > +
> > > +	if (!list_empty(&node->parent_val)) {
> > > +		chain = list_first_entry(&node->parent_val, struct callchain_list, list);
> > > +		chain->has_children = rb_prev(&node->rb_node) || rb_next(&node->rb_node);
> > > +
> > > +		chain = list_first_entry(&node->val, struct callchain_list, list);
> > > +		chain->has_children = false;
> > 
> > I'm a bit puzzled with this, can't we rewind through the parents on printing or adding
> > to the flat rbtree instead of having this parent_val field?
> 
> Yes, this code is to simplify things on parent nodes.  Maybe we could
> go up to parents and print the callchain list there as you said.
> 
> However, problem I think is how to handle 'has_children' information
> on parents.  That info controls folding status of each callchain.  As
> the info is in the struct callchain_list and flat or folded callchain
> mode require the info should be in the top-most entry, I cannot share
> entries in parent nodes.
> 
> Thus I simply copied callchain lists in parents to leaf nodes.  Yes,
> it will consume some memory but can simplify the code.

I haven't done any measuring, but I'm noticing that 'perf top -g' is
showing more warnings about not being able to process events fast enough
and so ends up losing events, I tried with --max-stack 16 and it helped,
this is just a heads up.

Perhaps my workstation workloads are gettning deeper callchains over
time, but perhaps this is the cost of processing callchains that is
increasing, I need to stop and try to quantify this.

We really need to look at reducing the overhead of processing
callchains.

- Arnaldo
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