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

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