perf backtraces off-by-1

perf backtraces off-by-1

[Arun Sharma]

Some of our language runtimes like to map IP addresses in perf backtrace 
to specific byte codes. The way things stand now, the addresses on the 
backtrace are return addresses, rather than the caller. I think this 
issue may be present for other unusual call/return sequences where the 
user may be more interested in the calling instruction rather than the 
instruction control flow would return to.

A simple hack such as the one below makes our JIT guys happy. But the
code is not right if there was an asynchronous transfer of control (eg:
signal handler or interrupt).

libunwind contains similar code, but has the additional info in the 
unwind information to recognize async control transfer.

Wondering if this has been discussed before. One option is to support 
this for user mode only, with code to detect signal frames. Any other ideas?

        -Arun

--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -296,6 +296,7 @@ int machine__resolve_callchain(struct machine *self, 
struct perf_evsel *evsel,
	u8 cpumode = PERF_RECORD_MISC_USER;
	unsigned int i;
	int err;
+	int async;

	callchain_cursor_reset(&evsel->hists.callchain_cursor);

@@ -322,6 +323,11 @@ int machine__resolve_callchain(struct machine 
*self, struct perf_evsel *evsel,
			continue;
		}

+		/* XXX: check if this was an async control transfer */
+		async = 0;
+                if (!async) {
+			ip--;
+		}
		al.filtered = false;
		thread__find_addr_location(thread, self, cpumode,
					   MAP__FUNCTION, ip, &al, NULL);

Re: perf backtraces off-by-1

[Peter Zijlstra]

On Fri, 2012-08-24 at 15:13 -0700, Arun Sharma wrote:
> 
> Wondering if this has been discussed before.

Not that I can recall.

>  One option is to support 
> this for user mode only, with code to detect signal frames. Any other
> ideas?
> 
I guess we'd need to see what that patch would look like... :-)

Re: perf backtraces off-by-1

[Arun Sharma]

On 8/26/12 9:10 AM, Peter Zijlstra wrote:
> On Fri, 2012-08-24 at 15:13 -0700, Arun Sharma wrote:
>
>>   One option is to support
>> this for user mode only, with code to detect signal frames. Any other
>> ideas?
>>
> I guess we'd need to see what that patch would look like... :-)
>

It used to look like this:

http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=commitdiff;h=92cc7fd78a5a79c4bb5f85bfb7d7fb025df9cd5a

These days we just look at dwarf augmentation string:

http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=blob;f=src/dwarf/Gfde.c;h=8659624b0320c514057861a259b6efe1b605bbf3;hb=HEAD#l189

  -Arun

Re: perf backtraces off-by-1

[Peter Zijlstra]

On Sun, 2012-08-26 at 10:52 -0700, Arun Sharma wrote:
> On 8/26/12 9:10 AM, Peter Zijlstra wrote:
> > On Fri, 2012-08-24 at 15:13 -0700, Arun Sharma wrote:
> >
> >>   One option is to support
> >> this for user mode only, with code to detect signal frames. Any other
> >> ideas?
> >>
> > I guess we'd need to see what that patch would look like... :-)
> >
> 
> It used to look like this:
> 
> http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=commitdiff;h=92cc7fd78a5a79c4bb5f85bfb7d7fb025df9cd5a

Hmm, that's not too bad, but a long stretch from pretty ;-)

How would you 'encode' this in the perf callchain data? 

> These days we just look at dwarf augmentation string:
> 
> http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=blob;f=src/dwarf/Gfde.c;h=8659624b0320c514057861a259b6efe1b605bbf3;hb=HEAD#l189

Right, except of course we don't have that in kernel..

BTW, are you in San Diego for LinuxCon/LPC etc?

Re: perf backtraces off-by-1

[Arun Sharma]

On 8/28/12 9:34 AM, Peter Zijlstra wrote:
>> It used to look like this:
>>
>> http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=commitdiff;h=92cc7fd78a5a79c4bb5f85bfb7d7fb025df9cd5a
>
> Hmm, that's not too bad, but a long stretch from pretty ;-)
>
> How would you 'encode' this in the perf callchain data?
>
>> These days we just look at dwarf augmentation string:
>>
>> http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=blob;f=src/dwarf/Gfde.c;h=8659624b0320c514057861a259b6efe1b605bbf3;hb=HEAD#l189
>
> Right, except of course we don't have that in kernel..
>

The ip-- transformation could happen in user space. The kernel doesn't 
have to know any of this :)

  -Arun