Re: [PATCH v6 1/6] perf report: properly handle branch count in match_chain

[Namhyung Kim]

Hi Arnaldo and Milian,

On Fri, Oct 20, 2017 at 10:39:35AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Fri, Oct 20, 2017 at 01:38:23PM +0200, Milian Wolff escreveu:
> > On Freitag, 20. Oktober 2017 12:21:35 CEST Milian Wolff wrote:
> > > On Donnerstag, 19. Oktober 2017 17:01:08 CEST Namhyung Kim wrote:
> > > > Hi Andi,
> > > > 
> > > > On Thu, Oct 19, 2017 at 06:55:19AM -0700, Andi Kleen wrote:
> > > > > On Thu, Oct 19, 2017 at 12:59:14PM +0200, Milian Wolff wrote:
> > > > > > On Donnerstag, 19. Oktober 2017 00:41:04 CEST Andi Kleen wrote:
> > > > > > > Milian Wolff <milian.wolff@xxxxxxxx> writes:
> > > > > > > > +static enum match_result match_address_dso(struct dso *left_dso,
> > > > > > > > u64
> > > > > > > > left_ip, +					   struct dso *right_dso, u64 right_ip)
> > > > > > > > +{
> > > > > > > > +	if (left_dso == right_dso && left_ip == right_ip)
> > > > > > > > +		return MATCH_EQ;
> > > > > > > > +	else if (left_ip < right_ip)
> > > > > > > > +		return MATCH_LT;
> > > > > > > > +	else
> > > > > > > > +		return MATCH_GT;
> > > > > > > > +}
> > > > > > > 
> > > > > > > So why does only the first case check the dso? Does it not matter
> > > > > > > for the others?
> > > > > > > 
> > > > > > > Either should be checked by none or by all.
> > > > > > 
> > > > > > I don't see why it should be checked. It is only required to prevent
> > > > > > two
> > > > > > addresses to be considered equal while they are not. So only the one
> > > > > > check is required, otherwise we return either LT or GT.
> > > > > 
> > > > > When the comparison is always in the same process (which I think
> > > > > is not the case) just checking the addresses is sufficient. If they are
> > > > > not then you always need to check the DSO and only compare inside the
> > > > > same DSO.
> > > > 
> > > > As far as I know, the node->ip is a relative address (inside a DSO).
> > > > So it should compare the dso as well even in the same process.
> > > 
> > > Sorry guys, I seem to be slow at understanding your review comments.
> > > 
> > > match_address_dso should impose a sort order on two relative addresses. The
> > > order should ensure that relative addresses in a different DSO are not
> > > considered equal. But if the DSOs are different, it doesn't matter whether
> > > we return LT or GT - or?
> > > 
> > > Put differently, how would you write this function to take care of the DSO
> > > in the other two branches? I.e. what to return if the DSOs are different -
> > > a MATCH_ERROR?
> > 
> > Thinking a bit more about this. Are you guys maybe hinting at my 
> > implementation breaking the strict ordering rules (is that the right word?). 
> > I.e. a < b && b > a iff a == b ? Potentially my implementation would break 
> > this assumption when the relative IPs are the same, but the DSO is different.
> > 
> > So is this what you want:
> > 
> > +static enum match_result match_address_dso(struct dso *left_dso, u64
> >  left_ip, +                                         struct dso *right_dso, u64 
> > right_ip)
> >  +{
> >  +       if (left_dso == right_dso && left_ip == right_ip)
> >  +               return MATCH_EQ;
> >  +       else if (left_dso < right_dso || left_ip < right_ip)
> >  +               return MATCH_LT;
> >  +       else
> >  +               return MATCH_GT;
> >  +}

How about

	if (left_dso != right_dso)
		return left_dso < right_dso ? MATCH_LT : MATCH_GT;
	else if (left_ip != right_ip)
		return left_ip < right_ip ? MATCH_LT : MATCH_GT;
	else
		return MATCH_EQ;

?


> 
> Why not do all in terms of absolute addresses? Comparing relative
> addresses seems nonsensical anyway.

???

It needs to compare symbols of callchains from different address
spaces (i.e. tasks) too.  We do the same when comparing symbols of
samples - please see sort__sym_cmp().


> Perhaps something like the patch
> below, and note that cnode->ip and node->ip already already are absolute
> addresses.

Only if it couldn't find a map?

Thanks,
Namhyung


> 
> diff --git a/tools/perf/util/callchain.c b/tools/perf/util/callchain.c
> index 35a920f09503..1ac3f4a5afab 100644
> --- a/tools/perf/util/callchain.c
> +++ b/tools/perf/util/callchain.c
> @@ -671,8 +671,6 @@ static enum match_result match_chain(struct callchain_cursor_node *node,
>  {
>  	struct symbol *sym = node->sym;
>  	u64 left, right;
> -	struct dso *left_dso = NULL;
> -	struct dso *right_dso = NULL;
>  
>  	if (callchain_param.key == CCKEY_SRCLINE) {
>  		enum match_result match = match_chain_strings(cnode->srcline,
> @@ -698,16 +696,14 @@ static enum match_result match_chain(struct callchain_cursor_node *node,
>  			return match_chain_strings(cnode->ms.sym->name,
>  						   node->sym->name);
>  
> -		left = cnode->ms.sym->start;
> -		right = sym->start;
> -		left_dso = cnode->ms.map->dso;
> -		right_dso = node->map->dso;
> +		left = cnode->ms.map->unmap_ip(cnode->ms.map, cnode->ms.sym->start);
> +		right = node->map->unmap_ip(node->map, sym->start);
>  	} else {
>  		left = cnode->ip;
>  		right = node->ip;
>  	}
>  
> -	if (left == right && left_dso == right_dso) {
> +	if (left == right) {
>  		if (node->branch) {
>  			cnode->branch_count++;
>