Re: [PATCH v6 1/7] perf/core: Define the common branch type classification

From: Jin, Yao
Date: Mon Jul 10 2017 - 04:16:40 EST

On 7/10/2017 2:05 PM, Michael Ellerman wrote:
Hi Jin Yao,

Sorry I haven't commented until now, but it got lost in the flood of

Never mind, it's no problem. :)

Just a few nit-picks below ...
Jin Yao <yao.jin@xxxxxxxxxxxxxxx> writes:

It is often useful to know the branch types while analyzing branch
data. For example, a call is very different from a conditional branch.

Currently we have to look it up in binary while the binary may later
not be available and even the binary is available but user has to take
some time. It is very useful for user to check it directly in perf

Perf already has support for disassembling the branch instruction
to get the x86 branch type.

To keep consistent on kernel and userspace and make the classification
more common, the patch adds the common branch type classification
in perf_event.h.
Most of the code and doc uses "branch" but then a few these are called
"jump". Can we just stick with "branch"?

PERF_BR_NONE : unknown
PERF_BR_JCC : conditional jump
PERF_BR_JMP : jump
PERF_BR_IND_JMP : indirect jump

PERF_BR_COND : conditional branch
PERF_BR_UNCOND : unconditional branch
PERF_BR_IND : indirect branch

Call and jump are all branches. If we want to figure out which one is jump and which one is call, we need the detail branch type definitions.

For example, if we only say "PERF_BR_IND", we could not know if it's an indirect jump or indirect call.
PERF_BR_IND_CALL : indirect call
PERF_BR_RET : return
PERF_BR_SYSRET : syscall return
PERF_BR_IRQ : hw interrupt/trap/fault
PERF_BR_INT : sw interrupt
I'm not sure what that means, I'm guessing on x86 it means someone
executed "int" ?

PERF_BR_IRQ is for hw interrupt and PERF_BR_INT is for sw interrupt.

PERF_BR_CALL/PERF_BR_IND_CALL and PERF_BR_RET are for function call (direct call and indirect call) and return.

PERF_BR_SYSCALL/PERF_BR_SYSRET are for syscall and syscall return.

Is that sufficiently useful to use up a bit? I think we only have 3

Do you means 3 bits? Each bit stands for one branch type? I guess what you mean is:

PERF_BR_COND : conditional branch
PERF_BR_UNCOND : unconditional branch
PERF_BR_IND : indirect branch

But 3 branch types are not enough for us.

PERF_BR_IRET : return from interrupt
PERF_BR_FAR_BRANCH: not generic far branch type
What is a "not generic far branch" ?

I don't know what that would mean on powerpc for example.

It's reserved for future using I think.

I think the only thing we have on powerpc that's commonly used and that
isn't covered above is branches that decrement a loop counter and then
branch based on the result.

It might be nice if we could separate those out from other conditional
branches. Whether it's worth using a bit for I'm not sure. Do other
arches have something similar?

Those branches do tend to be "backward conditional", so that may be
sufficient. But backward conditional also includes if bodies that have
been moved out of line and then branch back to the main body of the


Sorry, I'm not familiar with powerpc arch. Or could you add the branch type which powerpc needs?

For backward conditional and forward conditional, we compute them in userspace according to the from/to addresses.

Jin Yao