On Wednesday, September 21, 2016 6:09:15 PM CEST Mathieu Desnoyers wrote:
> ----- On Sep 20, 2016, at 4:34 PM, Milian Wolff milian.wo...@kdab.com wrote:
> > On Montag, 12. September 2016 16:24:04 CEST Mathieu Desnoyers wrote:
> >> ----- On Sep 12, 2016, at 11:40 AM, Milian Wolff milian.wo...@kdab.com
> > 
> > wrote:
> >> > On Monday, September 12, 2016 3:03:04 PM CEST Mathieu Desnoyers wrote:
> >> >> ----- On Sep 6, 2016, at 3:00 PM, Milian Wolff milian.wo...@kdab.com
> > 
> > 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 | milian.wo...@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts

Attachment: smime.p7s
Description: S/MIME cryptographic signature

_______________________________________________
lttng-dev mailing list
lttng-dev@lists.lttng.org
https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

Reply via email to