perf backtraces off-by-1

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

@@ -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);

Not that I can recall.

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

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

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

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

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

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

-Arun