Re: [PATCH -tip v3 0/6] perf: Introduce branch sub commands

[Akihiro Nagai <akihiro.nagai.hw@hitachi.com>]

(2011/03/30 23:46), Frederic Weisbecker wrote:
<snip>
>> 'perf branch trace' can parse and analyze recorded BTS log and print various
>> information of execution path. This command can show address, pid, command name,
>> function+offset, file path of elf.
>> You can choose the printed information with option.
>>
>> Example: 'perf branch trace'
>> function+offset
>> irq_return+0x0  =>  _start+0x0
>> irq_return+0x0  =>  _start+0x0
>> _start+0x3      =>  _dl_start+0x0
>> irq_return+0x0  =>  _dl_start+0x0
>> irq_return+0x0  =>  _dl_start+0x26
>> irq_return+0x0  =>  _dl_start+0x2d
>
> These results are a bit surprising. May be we can
> jump once from irq_return to _start, in the first schedule()
> of a new task perhaps, but thereafter I would expect
> further jumps not to happen from irq_return, but rather
> from _start. When we have x as a destination in line n, then
> I would expect to have x as a source in n + 1.
Agree with the opinion "irq_start" surprising users.
However, I think it is not a better solution that uses a
previous destination as a next source.
Because, users want to know what happen in userspace and,
do not want to know interrupts from kernel.

I think the better solution is to implement the filter that
eliminate the record including kernel functions from output.
For example, leading example will be filtered like this.

_start+0x3      =>  _dl_start+0x0

In the future, I think the solution is available that using BTS records
with trace event like irq:irq_handler_entry to analyze interrupt.
However, to do it, we need to fix perf.

>
> Also we are supposed to only trace BTS in userspace, but
> perhaps, if we are interrupted, after the execution of the iret instruction,
> BTS considers the following jump "iret ->  interrupted inst" as a branch
> in userspace. After all it makes sense, it is a jump in userspace.
>
> So BTS, because of the way it defines a jump inside userspace,
> traces irq returns but not irq entries, that would explain the trace
> you gave as an example.
>
> I suspect we want to filter irq returns. ie: if the source comes
> from the kernel, then filter it by default. And then we can later
> think about an option to enable interrupt return tracing if
> people want them.
Agree.
I will implement the option that enable/disable the filter.

>
> Thanks.
Thank you for your advise.