[PATCH perf/core 00/22] perf refcnt debugger API and fixes

Masami Hiramatsu
2015-12-09T02:18:47Z

Hi Arnaldo,

Here is a series of patches for perf refcnt debugger and
some fixes.

In this series I've replaced all atomic reference counters
with the refcnt interface, including dso, map, map_groups,
thread, cpu_map, comm_str, cgroup_sel, thread_map, and
perf_mmap.

refcnt debugger (or refcnt leak checker)
===============

At first, note that this change doesn't affect any compiled
code unless building with REFCNT_DEBUG=1 (see macros in
refcnt.h). So, this feature is only enabled in the debug binary.
But before releasing, we can ensure that all objects are safely
reclaimed before exit in -rc phase.

To use the refcnt debugger, you just build a perf binary with
REFCNT_DEBUG=1 as follows;
----
# make REFCNT_DEBUG=1
----
And run the perf command. If the refcnt debugger finds leaks,
it shows the summary of the bugs. Note that if the command does
not use stdio, it doesn't show anything because refcnt debugger
uses pr_debug. Please use --stdio option in such case.

E.g. with the first 13 patches, perf top shows that many objects
are leaked.
----
# ./perf top --stdio
q
exiting.
REFCNT: BUG: Unreclaimed objects found.
REFCNT: Total 3595 objects are not reclaimed.
"map" leaks 3334 objects
"dso" leaks 231 objects
"thread" leaks 9 objects
"comm_str" leaks 13 objects
"map_groups" leaks 8 objects
To see all backtraces, rerun with -v option
----
You can also dump all the backtrace data with -v option, but I
don't recommend you to do it on your console, because it will
be very very long (for example, above dumps 40MB text logs
on your console).

Instead, you can use PERF_REFCNT_DEBUG_FILTER env. var. to focus
on one object, and also use "2>" to redirect stderr output to file.
E.g.
----
# PERF_REFCNT_DEBUG_FILTER=map ./perf top --stdio -v 2> refcnt.log
q
exiting.
# less refcnt.log
mmap size 528384B
Looking at the vmlinux_path (8 entries long)
Using /lib/modules/4.3.0-rc2+/build/vmlinux for symbols
REFCNT: BUG: Unreclaimed objects found.
==== [0] ====
Unreclaimed map@0x2157dc0
Refcount +1 => 1 at
...
./perf() [0x4226fd]
REFCNT: Total 3229 objects are not reclaimed.
"map" leaks 3229 objects
----

Bugfixes
========

In this series I've also tried to fix some object leaks in perf top
and perf stat.
After applying this series, this reduced (not vanished) to 1/5.
----
# ./perf top --stdio
q
exiting.
REFCNT: BUG: Unreclaimed objects found.
REFCNT: Total 866 objects are not reclaimed.
"dso" leaks 213 objects
"map" leaks 624 objects
"comm_str" leaks 12 objects
"thread" leaks 9 objects
"map_groups" leaks 8 objects
To see all backtraces, rerun with -v option
----

Actually, I'm still not able to fix all of the bugs. It seems that
hists has a bug that hists__delete_entries doesn't delete all the
entries because some entries are on hists->entries but others on
hists->entries_in. And I'm not so sure about hists.c.
Arnaldo, would you have any idea for this bug?

General refcnt miscodings
=========================

BTW, while applying this change, I've found that there are refcnt
coding mismatches in those code and most of the bugs come from those
mismatches.

- The reference counter can be initialized by 0 or 1.
- If 0 is chosen, caller have to get it and free it if failed to
register appropriately.
- If 1 is chosen, caller doesn't need to get it, but when exits the
caller, it has to put it. (except for returning the object itself)
- The application should choose either one as its policy, to avoid
confusion.

perf tools mixes it up (moreover, it initializes 2 in a case) and
caller usually forgets to put it (it is not surprising, because too
many "put" usually cause SEGV by accessing freed object.)

As far as I can see, cgroup_sel, perf_mmap(this is initialized 0 or 2...),
thread, and comm_str are initialized by 0. Others are initialized by 1.

So, I'd like to suggest that we choose one policy and cleanup the code.
I recommend to use init by 1 policy, because anyway caller has to get
the refcnt. Suppose the below code;

----
obj__new() {
obj = zalloc(sizeof(*obj));
refcnt__init(obj, refcnt, 0);
return obj;
}

caller() {
obj = obj__new();
if (parent__add_obj(parent, obj) != SUCCESS) {
free(obj);
}
}
----

At first glance, this looks good. However, if the parent__add_obj() once
gets the obj(refcnt => 1) and fails to add it to parent list by some reason,
it should put the obj(refcnt => 0). This means the obj is already freed at
that point.

Then, caller() shouldn't free obj in error case? No, because parent__add_obj()
can fail before getting the obj :(. Maybe we can handle it by checking return
code, but it is ugly.

If we choose "init by 1", caller always has to put it before returning. But
the coding rule becomes simpler.
----
caller() {
obj = obj__new();
if (parent__add_obj(parent, obj) != SUCCESS) {
ret = errorcode;
}
obj__put(obj);
return ret;
}
----

Thank you,

---

Masami Hiramatsu (22):
[v2] perf refcnt: Introduce generic refcount APIs with debug feature
perf refcnt: Use a hash for refcnt_root
perf refcnt: Add refcnt debug filter
perf refcnt: refcnt shows summary per object
perf: make map to use refcnt
perf: Make dso to use refcnt for debug
perf: Make map_groups to use refcnt
perf: Make thread uses refcnt for debug
perf: Make cpu_map to use refcnt for debug
perf: Make comm_str to use refcnt for debug
perf: Make cgroup_sel to use refcnt for debug
perf: Make thread_map to use refcnt for debug
perf: Make perf_mmap to use refcnt for debug
perf: Fix dso__load_sym to put dso
perf: Fix map_groups__clone to put cloned map
perf: Fix __cmd_top and perf_session__process_events to put the idle thread
perf: Fix __machine__addnew_vdso to put dso after add to dsos
perf stat: Fix cmd_stat to release cpu_map
perf: fix hists_evsel to release hists
perf: Fix maps__fixup_overlappings to put used maps
perf: Fix machine.vmlinux_maps to make sure to clear the old one
perf: Fix write_numa_topology to put cpu_map instead of free

tools/perf/builtin-stat.c | 11 ++
tools/perf/builtin-top.c | 6 +
tools/perf/config/Makefile | 5 +
tools/perf/util/Build | 1
tools/perf/util/cgroup.c | 11 +-
tools/perf/util/comm.c | 8 +
tools/perf/util/cpumap.c | 33 +++---
tools/perf/util/dso.c | 7 +
tools/perf/util/evlist.c | 8 +
tools/perf/util/header.c | 2
tools/perf/util/hist.c | 10 ++
tools/perf/util/machine.c | 5 +
tools/perf/util/map.c | 15 ++-
tools/perf/util/map.h | 5 +
tools/perf/util/refcnt.c | 229 ++++++++++++++++++++++++++++++++++++++++++
tools/perf/util/refcnt.h | 70 +++++++++++++
tools/perf/util/session.c | 10 ++
tools/perf/util/symbol-elf.c | 2
tools/perf/util/thread.c | 7 +
tools/perf/util/thread_map.c | 28 +++--
tools/perf/util/vdso.c | 2
21 files changed, 420 insertions(+), 55 deletions(-)
create mode 100644 tools/perf/util/refcnt.c
create mode 100644 tools/perf/util/refcnt.h

--
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@hitachi.com