Messages in this thread |  | | | Date | Tue, 24 Jun 2008 04:20:10 +0400 | | From | Alexey Dobriyan <> | | Subject | Re: [RFC] Tracepoint proposal |
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On Sun, Jun 22, 2008 at 02:27:05PM -0400, Mathieu Desnoyers wrote: > * Alexey Dobriyan (adobriyan@gmail.com) wrote: > > On Sun, Jun 22, 2008 at 01:11:35PM -0400, Mathieu Desnoyers wrote: > > > Tracepoint proposal > > > > > > - Tracepoint infrastructure > > > - In-kernel users > > > - Complete typing, verified by the compiler > > > - Dynamically linked and activated > > > > > > - Marker infrastructure > > > - Exported API to userland > > > - Basic types only > > > > > > - Dynamic vs static > > > - In-kernel probes are dynamically linked, dynamically activated, connected to > > > tracepoints. Type verification is done at compile-time. Those in-kernel > > > probes can be a probe extracting the information to put in a marker or a > > > specific in-kernel tracer such as ftrace. > > > - Information sinks (LTTng, SystemTAP) are dynamically connected to the > > > markers inserted in the probes and are dynamically activated. > > > > > > - Near instrumentation site vs in a separate tracer module > > > > > > A probe module, only if provided with the kernel tree, could connect to internal > > > tracing sites. This argues for keeping the tracepoing probes near the > > > instrumentation site code. However, if a tracer is general purpose and exports > > > typing information to userspace through some mechanism, it should only export > > > the "basic type" information and could be therefore shipped outside of the > > > kernel tree. > > > > > > In-kernel probes should be integrated to the kernel tree. They would be close to > > > the instrumented kernel code and would translate between the in-kernel > > > instrumentation and the "basic type" exports. Other in-kernel probes could > > > provide a different output (statistics available through debugfs for instance). > > > ftrace falls into this category. > > > > > > Generic or specialized information "sinks" (LTTng, systemtap) could be connected > > > to the markers put in tracepoint probes to extract the information to userspace. > > > They would extract both typing information and the per-tracepoint execution > > > information to userspace. > > > > > > Therefore, the code would look like : > > > > > > kernel/sched.c: > > > > > > #include "sched-trace.h" > > > > > > schedule() > > > { > > > ... > > > trace_sched_switch(prev, next); > > > ... > > > } > > > > Once this is accepted you're going to add hundreds of such calls to every > > core subsystem, right? > > > > The LTTng instrumentation has about 125 of such calls. Tests have > revealed that adding such dormant tracepoints to the kernel often > increase kernel performances rather than decreasing it (see the ia64 > benchmarks posted on lkml a few weeks ago).
We're not adding this for performance increase, you do realize this?
> The core subsystem maintainers are being involved in the process.
NAK this from proc if you about this.
> Actually, marking up the source code has the interesting effect of > letting knowledgeable people influence the trace point decisions.
I'd say that maximum source code overhead any tracing facility should be allowed is "__xxx" annotation at very start of function definition. Anything beyond should be rejected and there are good reasons for that.
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