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

    Description:

    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);
    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.
    - 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(-)

    --
    1.8.3.1

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