linux-kernel - Re: [PATCH v6 13/18] perf: add support for taken branch sampling to perf report

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Date:	Wed, 22 Feb 2012 17:18:10 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Roberto Agostino Vitillo <ravitillo@....gov>,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	acme@...hat.com, robert.richter@....com, ming.m.lin@...el.com,
	andi@...stfloor.org, asharma@...com, vweaver1@...s.utk.edu,
	khandual@...ux.vnet.ibm.com, dsahern@...il.com
Subject: Re: [PATCH v6 13/18] perf: add support for taken branch sampling to
 perf report

On Wed, Feb 22, 2012 at 12:15 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Stephane Eranian <eranian@...gle.com> wrote:
>
>> From: Roberto Agostino Vitillo <ravitillo@....gov>
>>
>> This patch adds support for taken branch sampling, i.e, the
>> PERF_SAMPLE_BRANCH_STACK feature to perf report. In other
>> words, to display histograms based on taken branches rather
>> than executed instructions addresses.
>>
>> The new option is called -b and it takes no argument. To
>> generate meaningful output, the perf.data must have been
>> obtained using perf record -b xxx ... where xxx is a branch
>> filter option.
>>
>> The output shows symbols, modules, sorted by 'who branches
>> where' the most often. The percentages reported in the first
>> column refer to the total number of branches captured and
>> not the usual number of samples.
>>
>> Here is a quick example.
>> Here branchy is simple test program which looks as follows:
>>
>> void f2(void)
>> {}
>> void f3(void)
>> {}
>> void f1(unsigned long n)
>> {
>>   if (n & 1UL)
>>     f2();
>>   else
>>     f3();
>> }
>> int main(void)
>> {
>>   unsigned long i;
>>
>>   for (i=0; i < N; i++)
>>    f1(i);
>>   return 0;
>> }
>>
>> Here is the output captured on Nehalem, if we are
>> only interested in user level function calls.
>>
>> $ perf record -b any_call,u -e cycles:u branchy
>>
>> $ perf report -b --sort=symbol
>>     52.34%  [.] main                   [.] f1
>>     24.04%  [.] f1                     [.] f3
>>     23.60%  [.] f1                     [.] f2
>>      0.01%  [k] _IO_new_file_xsputn    [k] _IO_file_overflow
>>      0.01%  [k] _IO_vfprintf_internal  [k] _IO_new_file_xsputn
>>      0.01%  [k] _IO_vfprintf_internal  [k] strchrnul
>>      0.01%  [k] __printf               [k] _IO_vfprintf_internal
>>      0.01%  [k] main                   [k] __printf
>
> Ok, nice feature.
>
> One detail needs to be fixed though, if someone does:
>
>  perf record -b ...
>
> then 'perf report' should *default* to the above branch stack
> output style, without having to specify -b again.
>
Fair enough.

I'll check how we could do that. It's not so obvious as the code
stands. I think we may need to add a new feature bit for that.
It would avoid having to sniff either the cmdline, the event desc
or worst the samples themselves.

> Having --branch/--no-branch present in perf report is fine if
> someone wants to force either direction, but the default
> absolutely must be picked up from the perf.data and should be
> the obvious behavior.
>
> Other than that it looks good to me, so if this detail is fixed
> (can be a delta patch on top of the existing series) and there's
> no problems with it I can pick it up for v3.4.
>
It'll be a delta patch.
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