----- 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 ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
