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
>
