Messages in this thread | | | Date | Sat, 9 May 2009 14:53:06 +0200 | Subject | Re: [RFC] convert ftrace syscall tracer to TRACE_EVENT() | From | Frédéric Weisbecker <> |
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2009/5/9 Ingo Molnar <mingo@elte.hu>: > > * Jason Baron <jbaron@redhat.com> wrote: > >> Hi, >> >> I've been thinking about converting the current ftrace syscall >> tracer to the TRACE_EVENT() macros. There are a few issues with >> the current syscall tracer approach: >> >> 1) It has to be enabled for all processes and all syscalls. By >> moving to TRACE_EVENT(), it can be enabled/disabled per tracepoint >> and can also make use of the generic tracing filters, such as >> "trace all process for pid x" >> >> 2) Other tracers can not tie into it, since its not tracepoint >> based. TRACE_EVENT() fixes this. >> >> 3) data formatting. The syscall tracer I don't believe understands >> all the various types for output formatting. By moving to >> TRACE_EVENT(), we can print out a more readible syscall trace. >> >> 4) The ftrace syscall tracer needs a new arch specific code for >> each architecture. By converting to TRACE_EVENT() we don't need >> any architecutre specific code. >> >> Other issues to consider: >> >> * Maintainence. The current syscall tracer automatically picks up >> new syscalls. The TRACE_EVENT() will be harder to initially set >> up. But once its done, syscalls are obviously not added often. So >> I don't think this will be too bad. >> >> * Performance. The current syscall tracer adds a >> 'test_thread_flag()' to syscall entry/exit. The TRACE_EVENT() >> would add a per-syscall global to check. So they are going to have >> different cache profiles...however, the tracepoint infrastructure >> is hopefully moving to the 'immediate' value work, which will make >> this more highly optimized. >> >> I've also tested the patch shown below (which uses, >> DECLARE_TRACE(), as a preliminary proof of concept), using >> getpid() in a loop, and tbench, and saw very small performance >> differences. Obviously we would have to do more extensive testing >> before deciding. >> >> Patch is pretty rough, but should give a rough sense of what the >> DECLARE_TRACE() type patch might look like... > > Yeah, i very much agree with the direction. (I've Cc:-ed Tom Zanussi > who also has expressed interest in this.) > > I'm not sure about the implementation as you've posted it though: > > Firstly, it adds two new tracepoints to every system call. That is > unnecessary - we already have the TIF flag based callbacks, and we > can use the existing syscall attributes table to get to tracepoints > - without slow down (or impacting) the fast path in any way.
Agreed, that's unnecessary because we already hook in ptrace without impacting the off-case.
> Secondly, we should reuse the information we get in SYSCALL_DEFINE, > to construct the TRACE_EVENT tracepoints directly - without having > to list all syscalls again in a separate file.
Indeed, that's not trivial though, but feasible. I'm not sure we can reuse the TRACE_EVENT macro directly inside SYSCALL_DEFINE. The resulting macro tempest effect that would occur confuses me and I have troubles to imagine the result.
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