Re: [PATCH v2] perf report: distinguish between inliners in the same function

[Milian Wolff <milian.wolff@kdab.com>]

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On Monday, May 15, 2017 3:21:58 AM CEST Namhyung Kim wrote:
> Hi Milian,
> 
> On Sun, May 14, 2017 at 08:10:50PM +0200, Milian Wolff wrote:
> > On Freitag, 12. Mai 2017 15:01:29 CEST Namhyung Kim wrote:
> > > On Fri, May 12, 2017 at 12:37:01PM +0200, Milian Wolff wrote:
> > > > On Mittwoch, 10. Mai 2017 07:53:52 CEST Namhyung Kim wrote:
> > > > > Hi,
> > > > 
> > > > > On Wed, May 03, 2017 at 11:35:36PM +0200, Milian Wolff wrote:
> > > > <snip>
> > > > 
> > > > > > +static enum match_result match_chain_srcline(struct
> > > > > > callchain_cursor_node
> > > > > > *node, +					     struct callchain_list *cnode)
> > > > > > +{
> > > > > > +	char *left = get_srcline(cnode->ms.map->dso,
> > > > > > +				 map__rip_2objdump(cnode->ms.map, cnode->ip),
> > > > > > +				 cnode->ms.sym, true, false);
> > > > > > +	char *right = get_srcline(node->map->dso,
> > > > > > +				  map__rip_2objdump(node->map, node->ip),
> > > > > > +				  node->sym, true, false);
> > > > > > +	enum match_result ret = match_chain_strings(left, right);
> > > > > 
> > > > > I think we need to check inlined srcline as well.  There might be a
> > > > > case that two samples have different addresses (and from different
> > > > > callchains) but happens to be mapped to a same srcline IMHO.
> > > > 
> > > > I think I'm missing something, but isn't this what this function
> > > > provides?
> > > > The function above is now being used by the match_chain_inliner
> > > > function
> > > > below.
> > > > 
> > > > Ah, or do you mean for code such as this:
> > > > 
> > > > ~~~~~
> > > > inline_func_1(); inline_func_2();
> > > > ~~~~~
> > > > 
> > > > Here, both branches could be inlined into the same line and the same
> > > > issue
> > > > would occur, i.e. different branches get collapsed into the first
> > > > match
> > > > for
> > > > the given srcline?
> > > > 
> > > > Hm yes, this should be fixed too.
> > > 
> > > OK.
> > > 
> > > > But, quite frankly, I think it just shows more and more that the
> > > > current
> > > > inliner support is really fragile and leads to lots of issues
> > > > throughout
> > > > the code base as the inlined frames are different from non-inlined
> > > > frames, but should most of the same be handled just like them.
> > > > 
> > > > So, maybe it's time to once more think about going back to my initial
> > > > approach: Make inlined frames code-wise equal to non-inlined frames,
> > > > i.e.
> > > > instead of requesting the inlined frames within match_chain, do it
> > > > outside
> > > > and create callchain_node/callchain_cursor instances (not sure which
> > > > one
> > > > right now) for the inlined frames too.
> > > > 
> > > > This way, we should be able to centrally add support for inlined
> > > > frames
> > > > and
> > > > all areas will benefit from it:
> > > > 
> > > > - aggregation by srcline/function will magically work
> > > > - all browsers will automatically display them, i.e. no longer need to
> > > > duplicate the code for inliner support in perf script, perf report
> > > > tui/
> > > > stdio/...
> > > > - we can easily support --inline in other tools, like `perf trace
> > > > --call-
> > > > graph`
> > > > 
> > > > So before I invest more time trying to massage match_chain to behave
> > > > well
> > > > for inline nodes, can I get some feedback on the above?
> > > 
> > > Fair enough.  I agree that it'd be better adding them as separate
> > > callchain nodes when resolving callchains.
> > 
> > Can you, or anyone else more involved with the current callchain code,
> > guide me a bit?
> > 
> > My previous attempt at doing this can be seen here:
> > https://github.com/milianw/linux/commit/
> > 71d031c9d679bfb4a4044226e8903dd80ea601b3
> > 
> > There are some issues with that. Most of it boils down to the question of
> > how to feed an inlined frame into a callchain_cursor_node. The latter
> > contains a symbol* e.g. and users of that class currently rely on using
> > the IP to find e.g. the corresponding srcline.
> > 
> > From what I can see, we either have to hack inline nodes in there, cf. my
> > original approach in the github repo above. Or, better, we would have to
> > (heavily?) refactor the callchain_cursor_node struct and the code
> > depending on it. What I have in mind would be something like adding two
> > members to this struct:
> > 
> > const char* funcname;
> > const char* srcline;
> > 
> > For non-inlined frames, the funcname aliases the `symbol->name` value, and
> > the srcline is queried as-needed. For inlined frames the values from the
> > inlined node struct are used. The inlined frames for a given code
> > location would all share the same symbol and ip.
> > 
> > Would that be OK as a path forward?
> 
> I think you'd be better adding (fake) dso and sym to keep the inline
> information.  The fake dso can be paired with the original dso and
> maintain a tree of (inlined) symbols.  You may need a fake map to
> point the fake dso then.  It seems a bit compilcated but that way the
> code will be more consistent and easier to handle (e.g. for caching
> and/or deletion) IMHO.

Can you expand on this please? How would that solve the problem of finding a 
function name or srcline for a given inline frame?

I.e.: the function name is, currently, part of the sym. So the fake dso/map 
would contain an internal, fake, string table which fake symbols could 
leverage for the function name?

Sounds like doable, but also sounds like *a lot* of work. And I don't see how 
that would solve the srcline situation: That one is queried on-demand based on 
the IP of a frame. I would say that inline frames should keep the IP of the 
first non-inlined frame. But that would make it impossible to find the srcline 
for the N'th inlined frame... Am I missing something?

> Also, for the children mode, callchain nodes should have enough
> information to create hist entries (but I'm not sure how to apply
> self periods for those inlined entries).

I'm not sure I'm following here either, but once inlined frames are normal 
callchain nodes, all browsers will simply support them out of the box, no? All 
of the aggregation and browsing features should just work™. We'd only need to 
patch the browsers for special usecases, like when we want to change the 
visuals to make it clear that a given frame was actually inlined.

Bye

-- 
Milian Wolff | milian.wolff@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts

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