[PATCH 0/7] Uprobes: Support SDT markers having reference count (semaphore)

From: Ravi Bangoria
Date: Wed Jun 06 2018 - 04:34:09 EST

Why RFC again:

This series is different from earlier versions[1]. Earlier series
implemented this feature in trace_uprobe while this has implemented
the logic in core uprobe. Few reasons for this:
1. One of the major reason was the deadlock between uprobe_lock and
mm->mmap inside trace_uprobe_mmap(). That deadlock was not easy to fix
because mm->mmap is not in control of trace_uprobe_mmap() and it has
to take uprobe_lock to loop over trace_uprobe list. More details can
be found at[2]. With this new approach, there are no deadlocks found
so far.
2. Many of the core uprobe function and data-structures needs to be
exported to make earlier implementation simple. With this new approach,
reference counter logic is been implemented in core uprobe and thus
no need to export anything.


Userspace Statically Defined Tracepoints[3] are dtrace style markers
inside userspace applications. Applications like PostgreSQL, MySQL,
Pthread, Perl, Python, Java, Ruby, Node.js, libvirt, QEMU, glib etc
have these markers embedded in them. These markers are added by developer
at important places in the code. Each marker source expands to a single
nop instruction in the compiled code but there may be additional
overhead for computing the marker arguments which expands to couple of
instructions. In case the overhead is more, execution of it can be
omitted by runtime if() condition when no one is tracing on the marker:

if (reference_counter > 0) {
Execute marker instructions;

Default value of reference counter is 0. Tracer has to increment the
reference counter before tracing on a marker and decrement it when
done with the tracing.

Currently, perf tool has limited supports for SDT markers. I.e. it
can not trace markers surrounded by reference counter. Also, it's
not easy to add reference counter logic in userspace tool like perf,
so basic idea for this patchset is to add reference counter logic in
the trace_uprobe infrastructure. Ex,[4]

# cat tick.c
for (i = 0; i < 100; i++) {
DTRACE_PROBE1(tick, loop1, i);
DTRACE_PROBE1(tick, loop2, i);
printf("hi: %d\n", i);

Here tick:loop1 is marker without reference counter where as tick:loop2
is surrounded by reference counter condition.

# perf buildid-cache --add /tmp/tick
# perf probe sdt_tick:loop1
# perf probe sdt_tick:loop2

# perf stat -e sdt_tick:loop1,sdt_tick:loop2 -- /tmp/tick
hi: 0
hi: 1
hi: 2
Performance counter stats for '/tmp/tick':
3 sdt_tick:loop1
0 sdt_tick:loop2
2.747086086 seconds time elapsed

Perf failed to record data for tick:loop2. Same experiment with this
patch series:

# ./perf buildid-cache --add /tmp/tick
# ./perf probe sdt_tick:loop2
# ./perf stat -e sdt_tick:loop2 /tmp/tick
hi: 0
hi: 1
hi: 2
Performance counter stats for '/tmp/tick':
3 sdt_tick:loop2
2.561851452 seconds time elapsed

- 'reference counter' is called as 'semaphore' in original Dtrace
(or Systemtap, bcc and even in ELF) documentation and code. But the
term 'semaphore' is misleading in this context. This is just a counter
used to hold number of tracers tracing on a marker. This is not really
used for any synchronization. So we are referring it as 'reference
counter' in kernel / perf code.
- This patches still has one issue. If there are multiple instances of
same application running and user wants to trace any particular
instance, trace_uprobe is updating reference counter in all instances.
This is not a problem on user side because instruction is not replaced
with trap/int3 and thus user will only see samples from his interested
process. But still this is more of a correctness issue. I'm working on
a fix for this.

[1] https://lkml.org/lkml/2018/4/17/23
[2] https://lkml.org/lkml/2018/5/25/111
[3] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
[4] https://github.com/iovisor/bcc/issues/327#issuecomment-200576506

Ravi Bangoria (7):
Uprobes: Simplify uprobe_register() body
Uprobes: Support SDT markers having reference count (semaphore)
Uprobes/sdt: Fix multiple update of same reference counter
trace_uprobe/sdt: Prevent multiple reference counter for same uprobe
Uprobes/sdt: Prevent multiple reference counter for same uprobe
Uprobes/sdt: Document about reference counter
perf probe: Support SDT markers having reference counter (semaphore)

Documentation/trace/uprobetracer.rst | 16 +-
include/linux/uprobes.h | 5 +
kernel/events/uprobes.c | 502 +++++++++++++++++++++++++++++++----
kernel/trace/trace.c | 2 +-
kernel/trace/trace_uprobe.c | 74 +++++-
tools/perf/util/probe-event.c | 39 ++-
tools/perf/util/probe-event.h | 1 +
tools/perf/util/probe-file.c | 34 ++-
tools/perf/util/probe-file.h | 1 +
tools/perf/util/symbol-elf.c | 46 +++-
tools/perf/util/symbol.h | 7 +
11 files changed, 643 insertions(+), 84 deletions(-)