On Wednesday, September 21, 2016 6:09:15 PM CEST Mathieu Desnoyers wrote: > ----- On Sep 20, 2016, at 4:34 PM, Milian Wolff [email protected] wrote: > > On Montag, 12. September 2016 16:24:04 CEST Mathieu Desnoyers wrote: > >> ----- On Sep 12, 2016, at 11:40 AM, Milian Wolff [email protected] > > > > wrote: > >> > On Monday, September 12, 2016 3:03:04 PM CEST Mathieu Desnoyers wrote: > >> >> ----- On Sep 6, 2016, at 3:00 PM, Milian Wolff [email protected] > > > > wrote: > >> >> > Hey all, > >> >> > > >> >> > where can I find more documentation on how to use sdt.h to add > >> >> > static > >> >> > tracepoints to user-land applications? If this is not the right > >> >> > place > >> >> > to > >> >> > ask, please refer to to elsewhere. > >> >> > >> >> Hi Milian, > >> > > >> > Hey Mathieu, > >> > > >> >> LTTng-UST offers a "tracepoint" instrumentation facility, which can > >> >> optionally emit "sdt.h" probe points too for compatibility with > >> >> SystemTAP. > >> >> > >> >> > I plan to upstream a collection of tracepoints to Qt, and possibly > >> >> > elsewhere. One problem I'm having right now is figuring out how to > >> >> > "design" the tracepoints such that they have minimal overhead. > >> >> > >> >> The main question here would be: do you want your instrumentation to > >> >> be > >> >> usable with LTTng-UST ? If yes, then you want the tracepoint > >> >> instrumentation facility. Else, if you only plan on using SystemTAP, > >> >> you > >> >> can use sdt.h instrumentation. > >> >> > >> >> > So my questions: > >> >> My answers will be about lttng-ust tracepoints. > >> > > >> > Thanks for the clarifications! Are the LTTNG-UST tracepoints also > >> > "zero- > >> > overhead" like the SystemTap once, i.e. put into a separate section and > >> > "only" one nop is added when tracing is disabled? I could not find such > >> > an information anywhere yet. > >> > >> The lttng-ust tracepoint mechanism comes from the design of the > >> Linux kernel tracepoints, which have proven to be unnoticeable > >> when disabled. There is one slight technicality though: > >> > >> The linux kernel tracepoints use the asm goto mechanism and code patching > >> to flip between a no-op and a jump to dynamically enable each tracepoint. > >> This is all very fine for the kernel. > >> > >> Now for user-space, the lttng-ust tracepoints rather use a conditional > >> branch to skip over the entire stack setup and the function call. There > >> are a few reasons for using a old-fashioned conditional branch: first, > >> there are no widely adopted multi-core, live code patching library > >> available in user-space that am I aware of. Second, even if we would > >> have one, we may not want to send SIGSTOP/SIGCONT to the traced process, > >> due to debugger interactions. Thirdly, doing code patching may not > >> interact will security features such as read-only code sections and > >> code checksumming. Finally, doing code patching of library and > >> executable code will trigger copy-on-write of the touched pages, which > >> will prevent sharing those pages between processes running the same > >> apps/libraries. All in all, the conditional branch does not seem like > >> a too bad alternative after all. > >> > >> If we look at the code generated by SystemTAP sdt.h probes (as well > >> as DTrace), they are actually more heavyweight than what has been > >> claimed by Sun's marketing department back in the days: it does > >> cost a stack setup of all the variables passed to the the probe, > >> and indeed the function call is no-op'd. > >> > >> The downside here is that all side-effects, and layout of the arguments > >> for the no-op'd call, are taken by the instrumented application, even > >> though tracing is not dynamically enabled. > >> > >> Therefore, the SDT mechanism is not a lightweight as is generally > >> claimed. It is not "just a single no-op per site", but rather a > >> function call stack setup and a no-op. > > > > Thanks a lot for the in-depth explanation Mathieu, much appreciated! From > > what I gather, sdt.h does allow to check whether a given trace point is > > enabled or not, no? I.e. via the semaphores. Is there any performance > > difference between checking the semaphore of a sdt.h tracepoint vs. doing > > the analogous check for lttng-ust tracepoints? > > Not sure what you refer to about "semaphores". And it's unclear how you > define "checking if active": is it to decide in the fast path whether to > trace or not, or in a slow path to query what is currently active ?
The fast path to check whether to trace or not:
$ cat probes.d
provider milian {
probe test();
};
$ dtrace -C -h -s probes.d -o probes.h
$ cat probes.h
/* Generated by the Systemtap dtrace wrapper */
#define _SDT_HAS_SEMAPHORES 1
#define STAP_HAS_SEMAPHORES 1 /* deprecated */
#include <sys/sdt.h>
/* MILIAN_TEST ( ) */
#if defined STAP_SDT_V1
#define MILIAN_TEST_ENABLED() __builtin_expect (test_semaphore, 0)
#define milian_test_semaphore test_semaphore
#else
#define MILIAN_TEST_ENABLED() __builtin_expect (milian_test_semaphore, 0)
#endif
__extension__ extern unsigned short milian_test_semaphore __attribute__
((unused)) __attribute__ ((section (".probes")));
#define MILIAN_TEST() \
DTRACE_PROBE (milian, test)
--
Milian Wolff | [email protected] | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
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