Thanks for the explanations. I currently feel that the effort to get such a thing working is not going to materialize in the timeframe required for my current work, and therefore, regrettably, I will have to work around this somehow. I am currently thinking of using a combination of Python and Cython to generate a wrapper for the babeltrace iterator. We'll see.
Thanks, Amit Margalit IBM XIV - Storage Reinvented XIV-NAS Development Team Tel. 03-689-7774 Fax. 03-689-7230 From: Jérémie Galarneau <[email protected]> To: Amit Margalit/Israel/IBM@IBMIL Cc: [email protected] Date: 07/21/2013 10:07 PM Subject: Re: [lttng-dev] Question about filtering bytecode Sent by: [email protected] On Sun, Jul 21, 2013 at 1:51 AM, Amit Margalit <[email protected]> wrote: > Hi, > > I was thinking more about a programmer's guide to using this mechanism. > Unfortunately there is no programmer's guide on filters, but feel free to ask about specifics. > What I'd like to do is to have Babeltrace filter events it is showing me > from an existing trace. > > Consider the following example: > > There are 3 types of tracepoints being captured (without filtering): > > A debug log message (occasionally) which includes a "debug_level" value > (1..10 with 1 being highest importance). > Function entry (via gcc -finstrument-functions or other instrumentation). > Function exit (similarly). > > During a 1-hour period, I have 2-3 GB of trace data captured. > The trace includes events from 4 different processes. > > > If I use Babeltrace to read the trace I will get an enormously big result in > text (~30-40GB at least). > > I would like to be able to do something like this: > > $ babeltrace --filter '$ctx.procname == "smbd" || debug_level < 3' > /local/lttng-traces/ust/uid/2140/64-bit > > This would then let me see any events from the "smbd" process (since i added > procname to the context using lttng add-context) or debug messages from all > processes where the debug level is 1 or 2. > That would be a neat feature! There are currently no plans to support filtering in Babeltrace, but that contribution would be welcome. lttng-ust's filtering code is licensed under LGPL v2.1 so it could be library-fied and used by the ctf-text plug-in. In the meantime, I guess you could work around this limitation with a combination of AWK and shell scripting... > Regarding linking - I am referring to the function > lttng_enabler_event_link_bytecode(), called from > lttng-ust/liblttng-ust/lttng-events.c around line 628. > Thanks for clarifying! Keep in mind that the filter's byte code is generated by the lttng binary without knowing the events' declarations. Thus, the byte code must go through a linking stage not completely unlike what is done by a regular linker. One of the objectives of the linking stage is setting the appropriate offsets to load an event's fields. The byte code is also "specialized" by resolving each field's type and making sure the appropriate implementation of each operator is used. For instance, the FILTER_OP_EQ (check for equality) op-code is not implemented the same way for integers, doubles and strings (see lttng_filter_specialize_bytecode in lttng-filter-specialize.c). You may want to have a look at _lttng_filter_event_link_bytecode (lttng-ust/liblttng-ust/lttng-filter.c, line 318) to see how each step of the linking process is implemented. Regards, Jérémie > Thanks, > > Amit Margalit > IBM XIV - Storage Reinvented > XIV-NAS Development Team > Tel. 03-689-7774 > Fax. 03-689-7230 > > > > From: Jérémie Galarneau <[email protected]> > To: Amit Margalit/Israel/IBM@IBMIL > Cc: [email protected] > Date: 07/18/2013 07:10 PM > Subject: Re: [lttng-dev] Question about filtering bytecode > Sent by: [email protected] > ________________________________ > > > > Hi Amit, > > You may find information on the Babeltrace API in babeltrace/doc/API.txt > The filtering mechanism used by the user space tracer is unrelated to > Babeltrace. > > As for the sequence of events, this is how I understand it. > > When invoking the "lttng" binary with --enable-event and a filter > option, a byte code is generated from the filtering rules > (lttng-tools/src/lib/lttng-ctl/lttng-ctl.c:704). This byte code is > then handed off to the session-daemon. > > Then, when an instrumented application is launched and connects to the > session-daemon, it receives the byte code while its events are being > enabled. To answer your question directly, that byte code will be > executed by the tracee when the event is "triggered" > (lttng-ust/liblttng-ust/lttng-filter-interpreter.c:175). > > I'm not sure I understand your question about filter/event linking. A > filter has to be linked to an event since its rules (expressions) are > applied on that event's values. Could you clarify if that's not what > you meant? > > Regards, > Jérémie > > On Thu, Jul 18, 2013 at 3:12 AM, Amit Margalit <[email protected]> wrote: >> Hi, >> >> I'm considering whether I can write a wrapper for the libbabeltrace ctf >> iterator which provides a filtered "next()" method. >> >> I've started reading the code in the hopes of understanding how to do >> this, >> and yet, I'd really appreciate a link to some ordered documentation, or - >> any other kind of explanation. >> >> For example, one thing I still don't understand is the exact sequence, and >> where the bytecode is actually executed? What is the meaning and purpose >> of >> "linking" the filter to an event. >> >> Thanks, >> >> Amit Margalit >> IBM XIV - Storage Reinvented >> XIV-NAS Development Team >> Tel. 03-689-7774 >> Fax. 03-689-7230 >> _______________________________________________ >> lttng-dev mailing list >> [email protected] >> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev >> > > > > -- > Jérémie Galarneau > EfficiOS Inc. > http://www.efficios.com > > -- Jérémie Galarneau EfficiOS Inc. http://www.efficios.com
_______________________________________________ lttng-dev mailing list [email protected] http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
