linux-kernel - Re: [PATCH] perf report: auto-detect branch stack sampling mode

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Date:	Fri, 24 Feb 2012 16:40:39 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	David Ahern <dsahern@...il.com>
Cc:	linux-kernel@...r.kernel.org, acme@...hat.com,
	peterz@...radead.org, mingo@...e.hu, ravitillo@....gov,
	khandual@...ux.vnet.ibm.com, asharma@...com,
	robert.richter@....com, ming.m.lin@...el.com,
	vweaver1@...s.utk.edu, andi@...stfloor.org
Subject: Re: [PATCH] perf report: auto-detect branch stack sampling mode

On Fri, Feb 24, 2012 at 4:31 PM, David Ahern <dsahern@...il.com> wrote:
> On 2/24/12 8:28 AM, Stephane Eranian wrote:
>>
>> On Fri, Feb 24, 2012 at 4:24 PM, David Ahern<dsahern@...il.com>  wrote:
>>>
>>> On 2/24/12 2:40 AM, Stephane Eranian wrote:
>>>>
>>>>
>>>>
>>>> This patch adds auto-detection of samples with taken branch stacks.
>>>> The auto-detection avoids having to specify the -b or --branch-stack
>>>> option on the cmdline.
>>>>
>>>> The patch adds a new feature bit HEADER_BRANCH_STACK to mark the
>>>> presence of branch stacks in samples.
>>>>
>>>> You can now do:
>>>> $ perf record -b any noploop 2
>>>> $ perf report
>>>> # Events: 8K cycles
>>>> #
>>>> # Overhead  Command  Source Shared Object        Source Symbol  Target
>>>> Shared Object       Target Symbol
>>>> # ........  .......  ....................  ...................
>>>>  ....................  ..................
>>>> #
>>>>     91.56%  noploop  noploop               [.] noploop
>>>>   noploop  [.] noploop
>>>>      0.42%  noploop  [kernel.kallsyms]     [k] __lock_acquire
>>>>  [kernel.kallsyms]  [k] __lock_acquire
>>>>
>>>>
>>>> To force regular reporting based on the instruction address:
>>>> $ perf report --no-branch-stack
>>>> #
>>>> # Events: 2K cycles
>>>> #
>>>> # Overhead  Command      Shared Object                           Symbol
>>>> # ........  .......  .................  ...............................
>>>> #
>>>>     92.03%  noploop  noploop            [.] noploop
>>>>      1.00%  noploop  [kernel.kallsyms]  [k] lock_acquire
>>>>
>>>>
>>>> Signed-off-by: Stephane Eranian<eranian@...gle.com>
>>>> ---
>>>>
>>>> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
>>>> index 1c49d4e..5e833a2 100644
>>>> --- a/tools/perf/builtin-record.c
>>>> +++ b/tools/perf/builtin-record.c
>>>> @@ -473,6 +473,9 @@ static int __cmd_record(struct perf_record *rec, int
>>>> argc, const char **argv)
>>>>        if (!have_tracepoints(&evsel_list->entries))
>>>>                perf_header__clear_feat(&session->header,
>>>> HEADER_TRACE_INFO);
>>>>
>>>> +       if (!rec->opts.branch_stack)
>>>> +               perf_header__clear_feat(&session->header,
>>>> HEADER_BRANCH_STACK);
>>>
>>>
>>>
>>> branch tracing is user requested on, so shouldn't feature default off and
>>> only be enabled when requested?
>>>
>> Well, what Ingo was suggesting is that perf report auto-detects whether or
>> not branch mode is necessary by looking at the perf.data file. Most likely
>> if you've recorded with -b, you are interested in a branch mode view
>> rather
>> that the instruction view (default). So all this does is elimintate the
>> need
>> to pass -b to perf report to enable branch mode.
>
>
> Right. My point is that HEADER_BRANCH_STACK feature should be off by default
> and only enabled by builtin-record when branch stack is requested. You have
> the reverse -- enabled by default and off in record if not requested.
>
No, I don't. Read the code carefully. The for loop sets all known feature bits.
Then, the ones not necessary or unused are turned off individually.

> David
>
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