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Subject[PATCH v3 0/9] trace_uprobe: Support SDT markers having reference count (semaphore)
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Userspace Statically Defined Tracepoints[1] 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,[2]

# cat tick.c
...
for (i = 0; i < 100; i++) {
DTRACE_PROBE1(tick, loop1, i);
if (TICK_LOOP2_ENABLED()) {
DTRACE_PROBE1(tick, loop2, i);
}
printf("hi: %d\n", i);
sleep(1);
}
...

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
^C
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
^C
Performance counter stats for '/tmp/tick':
3 sdt_tick:loop2
2.561851452 seconds time elapsed


Note:
- '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.

v3 changes:
- [PATCH v3 6/9] Fix build failure.
- [PATCH v3 6/9] Move uprobe_mmap_callback() after no_uprobe_events()
check. Actually, it should be moved after MMF_HAS_UPROBES as well.
But current implementation is sub-optimal. 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. Once this gets fixed, we can
move uprobe_mmap_callback() call after MMF_HAS_UPROBES.
- [PATCH v3 7/9] Remove mmu_notifier. Instead, use callback from
uprobe_clear_state(). Again, uprobe_clear_state_callback() should be
moved after MMF_HAS_UPROBES. But that should be done when move
uprobe_mmap_callback() first.
- [PATCH v3 7/9] Properly handle error cases for sdt_increment_ref_ctr()
and trace_uprobe_mmap().
- [PATCH v3 9/9] Show warning if kernel doesn't support ref_ctr logic
and user tries to use it. Also, return error in this case instead
of adding entry in uprobe_events.
- [PATCH v3 9/9] Don't check kernel ref_ctr support while adding files
into buildid-cache.

v2 can be found at:
https://lkml.org/lkml/2018/4/4/127

v2 changes:
- [PATCH v2 3/9] is new. build_map_info() has a side effect. One has
to perform mmput() when he is done with the mm. Let free_map_info()
take care of mmput() so that one does not need to worry about it.
- [PATCH v2 6/9] sdt_update_ref_ctr(). No need to use memcpy().
Reference counter can be directly updated using normal assignment.
- [PATCH v2 6/9] Check valid vma is returned by sdt_find_vma() before
incrementing / decrementing a reference counter.
- [PATCH v2 6/9] Introduce utility functions for taking write lock on
dup_mmap_sem. Use these functions in trace_uprobe to avoid race with
fork / dup_mmap().
- [PATCH v2 6/9] Don't check presence of mm in tu->sml at decrement
time. Purpose of maintaining the list is to ensure increment happen
only once for each {trace_uprobe,mm} tuple.
- [PATCH v2 7/9] v1 was not removing mm from tu->sml when process
exits and tracing is still on. This leads to a problem if same
address gets used by new mm. Use mmu_notifier to remove such mm
from the list. This guarantees that all mm which has been added
to tu->sml will be removed from list either when tracing ends or
when process goes away.
- [PATCH v2 7/9] Patch description was misleading. Change it. Add
more generic python example.
- [PATCH v2 7/9] Convert sml_rw_sem into mutex sml_lock.
- [PATCH v2 7/9] Use builtin linked list in sdt_mm_list instead of
defining it's own pointer chain.
- Change the order of last two patches.
- [PATCH v2 9/9] Check availability of ref_ctr_offset support by
trace_uprobe infrastructure before using it. This ensures newer
perf tool will still work on older kernels which does not support
trace_uprobe with reference counter.
- Other changes as suggested by Masami, Oleg and Steve.

v1 can be found at:
https://lkml.org/lkml/2018/3/13/432

[1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
[2] https://github.com/iovisor/bcc/issues/327#issuecomment-200576506
[3] https://lkml.org/lkml/2017/12/6/976

Oleg Nesterov (1):
Uprobe: Move mmput() into free_map_info()

Ravi Bangoria (8):
Uprobe: Export vaddr <-> offset conversion functions
mm: Prefix vma_ to vaddr_to_offset() and offset_to_vaddr()
Uprobe: Rename map_info to uprobe_map_info
Uprobe: Export uprobe_map_info along with
uprobe_{build/free}_map_info()
trace_uprobe: Support SDT markers having reference count (semaphore)
trace_uprobe/sdt: Fix multiple update of same reference counter
trace_uprobe/sdt: Document about reference counter
perf probe: Support SDT markers having reference counter (semaphore)

Documentation/trace/uprobetracer.txt | 16 ++-
include/linux/mm.h | 12 ++
include/linux/uprobes.h | 20 +++
kernel/events/uprobes.c | 90 +++++++-----
kernel/trace/trace.c | 2 +-
kernel/trace/trace_uprobe.c | 271 ++++++++++++++++++++++++++++++++++-
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 +
12 files changed, 472 insertions(+), 67 deletions(-)

--
1.8.3.1

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