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

[Frederic Weisbecker]

On Thu, Mar 24, 2011 at 08:31:37PM +0900, Akihiro Nagai wrote:
> Hi,
> 
> This patch series provides the commands 'perf branch record' and 'perf branch trace'
> version 3. These commands record and analyze a BTS (Branch Trace Store) log.
> And, they provide the interface to use BTS log for application developers.
> 
> BTS is a facility of Intel x86 processors, which records the address of
> 'branch to/from' on every branch/jump instruction and interrupt.
> This facility is very useful for developers to test their software,
> for example, coverage test, execution path analysis, dynamic step count ...etc.
> These test tools have a big advantage, which user doesn't have to modify target
> executable binaries, because the BTS is a hardware feauture.
> 
> But, there are few applications using BTS. Reasons I guess are ...
>  - Few people know what BTS is.
>  - Few people know how to use BTS on Linux box.
>  - It's hard to analyze the BTS log because it includes just a series of addresses.
> 
> So, I want to provide a user-friendly interface to BTS for application developers.
> 
> 
>  About new sub commands
> ========================
> 'perf branch record' provides an easy way to record BTS log.
> Usage is 'perf branch record <command>'.  This command is just an alias to
> 'perf record -e branches:u -c 1 <command>'. But, new one is more simple and
> more intuitive.
> 
> '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.

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.

Thanks.
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