Re: [PATCH v9 0/4] Uprobes: Support SDT markers having reference count (semaphore)

[Song Liu]

I am testing the patch set with the following code:

#include <stdio.h>
#include <unistd.h>

volatile short semaphore = 0;

int for_uprobe(int c)
{
        printf("%d\n", c + 10);
        return c + 1;
}

int main(int argc, char *argv[])
{
        for_uprobe(argc);
        while (1) {
                sleep(1);
                printf("semaphore %d\n", semaphore);
        }
}

I created a uprobe on function for_uprobe(), that uses semaphore as
reference counter:

  echo "p:uprobe_1 /root/a.out:0x49a(0x1036)" >> uprobe_events

I found that if I start a.out before enabling the uprobe, the output
of a.out is like:

root@virt-test:~# ~/a.out
11
semaphore 0
semaphore 0
semaphore 2      <<<  when the uprobe is enabled
semaphore 2
semaphore 2
semaphore 2
semaphore 2
semaphore 0     <<< when the uprobe is disabled
semaphore 0
semaphore 0

I am not quite sure whether the value should be 1 or 2, but it works.

However, if I start a.out AFTER enabling the uprobe, there is something wrong:

root@virt-test:~# ~/a.out
11
semaphore 0       <<< this should be non-zero, as the uprobe is already enabled
semaphore 0
semaphore 0
semaphore 0
semaphore 0       <<< disabling the uprobe, the value is still 0,
                            <<<  get warning "ref_ctr going negative"
at the same time
semaphore 0
semaphore 0
semaphore 1       <<< enable uprobe again, getting 1 this time
semaphore 1
semaphore 1

I haven't spent much time debugging this, but I guess there is
something wrong on the mmap path?

Thanks,
Song

On Sun, Aug 19, 2018 at 9:42 PM, Ravi Bangoria
<ravi.bangoria@xxxxxxxxxxxxx> wrote:
> v8 -> v9:
>  - Rebased to rostedt/for-next (Commit bb730b5833b5 to be precise)
>  - Not including first two patches now. They are already pulled by
>    Steven.
>  - Change delayed_uprobe_remove() function as suggested by Oleg
>  - Dump inode, offset, ref_ctr_offset, mm etc if we fail to update
>    reference counter.
>  - Rename delayed_uprobe_install() to delayed_ref_ctr_inc()
>  - Use 'short d' (delta) in update_ref_ctr() in place of 'bool
>    is_register'.
>
> v8: https://lkml.org/lkml/2018/8/9/81
>
> Future work:
>  - Optimize uprobe_mmap()->delayed_ref_ctr_inc() by making
>    delayed_uprobe_list per mm.
>
> Description:
> 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 a 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
>
> [1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
> [2] https://github.com/iovisor/bcc/issues/327#issuecomment-200576506
>
> Ravi Bangoria (4):
>   Uprobes: Support SDT markers having reference count (semaphore)
>   Uprobes/sdt: Prevent multiple reference counter for same uprobe
>   trace_uprobe/sdt: Prevent multiple reference counter for same uprobe
>   perf probe: Support SDT markers having reference counter (semaphore)
>
>  include/linux/uprobes.h       |   5 +
>  kernel/events/uprobes.c       | 278 ++++++++++++++++++++++++++++++++++++++++--
>  kernel/trace/trace.c          |   2 +-
>  kernel/trace/trace_uprobe.c   |  75 +++++++++++-
>  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 ++
>  10 files changed, 453 insertions(+), 35 deletions(-)
>
> --
> 2.14.4
>