Re: [PATCH v6 0/6] Uprobes: Support SDT markers having reference count (semaphore)

[Ravi Bangoria]

Oleg, Srikar, Masami, Song,

Any feedback please? :)

Thanks,
Ravi

On 07/16/2018 02:17 PM, Ravi Bangoria wrote:
> 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
> 
> 
> v6 changes:
>  - Do not export struct uprobe outside of uprobe.c. Instead, use
>    container_of(arch_uprobe) to regain uprobe in uprobe_write_opcode().
>  - Allow 0 as a special value for reference counter _offset_. I.e.
>    two uprobes, one having ref_ctr_offset=0 and the other having
>    non-zero ref_ctr_offset can coexists.
>  - If vma holding reference counter is not present while patching an
>    instruction, we add that uprobe in delayed_uprobe_list. When
>    appropriate mapping is created, we increment the reference counter
>    and remove uprobe from delayed_uprobe_list. While doing all this,
>    v5 was searching for all such uprobes in uprobe_tree which is not 
>    require. Also, uprobes are stored in rbtree with inode+offset as
>    the key and v5 was searching uprobe based on inode+ref_ctr_offset
>    which is wrong too. Fix this by directly looing on delayed_uprobe_list.
>  - Consider VM_SHARED vma as invalid for reference counter. Return
>    false from valid_ref_ctr_vma() if vma->vm_flags has VM_SHARED set.
>  - No need to use FOLL_FORCE in get_user_pages_remote() while getting
>    a page to update reference counter. Remove it.
>  - Do not mention usage of reference counter in Documentation/.
> 
> 
> v5 can be found at: https://lkml.org/lkml/2018/6/28/51
> 
> Ravi Bangoria (6):
>   Uprobes: Simplify uprobe_register() body
>   Uprobe: Additional argument arch_uprobe to uprobe_write_opcode()
>   Uprobes: Support SDT markers having reference count (semaphore)
>   trace_uprobe/sdt: Prevent multiple reference counter for same uprobe
>   Uprobes/sdt: Prevent multiple reference counter for same uprobe
>   perf probe: Support SDT markers having reference counter (semaphore)
> 
>  arch/arm/probes/uprobes/core.c |   2 +-
>  arch/mips/kernel/uprobes.c     |   2 +-
>  include/linux/uprobes.h        |   7 +-
>  kernel/events/uprobes.c        | 358 ++++++++++++++++++++++++++++++++++++-----
>  kernel/trace/trace.c           |   2 +-
>  kernel/trace/trace_uprobe.c    |  78 ++++++++-
>  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, 503 insertions(+), 74 deletions(-)
>