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 v5 changes: - Fix build failure. 'struct uprobe' was local to uprobe.c file and I was using it in some arch specific code which caused a build failure. Added new patch [PATCH v5 1/10] to fix it. v4 changes: - Previous version moved the implementation from trace_uprobe to core uprobe. But it had some issues. I've fixed them in this version. To cut a long story short, reference counter increment/decrement is tied to instruction patching. Whenever instruction gets patched, we update the reference counter. Now, what if vma holding reference counter is not present while patching an instruction? To overcome this issue, we will add such uprobe to delayed_uprobe list. Whenever process maps the region holding the reference counter, we increment it and remove the uprobe from delayed_uprobe list. - Until last version, we were incrementing reference counter for each uprobe_consumer. That isn't true now. With this implementation, we increment and decrement the counter only once. This is fine because we increment the counter when we find first valid consumer and we decrement it when last consumer is going away. (For a tiny binary, multiple vmas maps to the same file portion. In such case, we are patching the instruction multiple time and thus we will increment / decrement the reference counter multiple time as well.) - Fortunately, there is no need to maintain sdt_mm_list now, because we are sure that the increment and decrement will always happen in sync. This make the implementation lot more simpler compared to earlier versions. v4 can be foud at: https://lkml.org/lkml/2018/6/19/1324 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. [1] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation [2] https://github.com/iovisor/bcc/issues/327#issuecomment-200576506 Ravi Bangoria (10): Uprobes: Move uprobe structure to uprobe.h Uprobes: Simplify uprobe_register() body Uprobe: Change set_swbp definition Uprobe: Change set_orig_insn definition Uprobe: Change uprobe_write_opcode definition 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 Uprobes/sdt: Document about reference counter perf probe: Support SDT markers having reference counter (semaphore) Documentation/trace/uprobetracer.rst | 16 +- arch/arm/probes/uprobes/core.c | 6 +- arch/mips/kernel/uprobes.c | 6 +- include/linux/uprobes.h | 37 +++- kernel/events/uprobes.c | 390 ++++++++++++++++++++++++++++------- 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 + 13 files changed, 542 insertions(+), 117 deletions(-) -- 2.14.4 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html