Messages in this thread | | | Date | Mon, 18 Sep 2006 13:47:42 -0400 | From | Mathieu Desnoyers <> | Subject | Re: MARKER mechanism, try 2 |
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* Ingo Molnar (mingo@elte.hu) wrote: > > * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote: > > > It supports 5 modes : > > > > - marker becomes nothing > > - marker calls printk > > - marker calls a tracer > > - marker puts a symbol (for kprobe) > > - marker puts a symbol and 5 NOPS for a jump probe. > > just go for 'nothing' and the 5-NOP variant, and please implement > support for it from within LTT, via a kprobe - if you want me to support > this stuff for upstream inclusion. If we support any static tracer mode > and LTT does not support the kprobe mode then we are back to square 1 > wrt. dependencies ... >
I am open to make LTTng support kprobes as a commodity (in fact, this point has been on the LTTng project roadmap for almost a year). But in no way does it solve the entire tracing problem. As an example, LTTng traces the page fault handler, when kprobes just can't instrument it.
I keep thinking that a complete marker mechanism must have the ability to be turned into function calls or inline functions when necessary.
Going further, we could think of a marker mechanism that would be aware of the "difficulty" level of the probe, so that even if CONFIG_KPROBELOG is selected, it would use a direct call or inlined function for probing the page fault handler.
i.e. :
"normal" (nothing, kprobe, jumpprobe, printk or tracer) MARK(eventname, "%d %s", myint, mystring);
"cannot be probed dynamically" (used in kprobes itself, page fault handler) (only nothing or tracer) MARK_NOPROBE(eventname, "%d %s", myint, mystring);
"cannot use printk" (used in scheduler, NMIs, wakeup, printk itself) (nothing, kprobe, jumpprobe or tracer) MARK_NOPRINT(eventname, "%d %s", myint, mystring);
Using the following table to select the mechanism :
Config/probe declaration | normal | noprobe | noprint ------------------------------------------------------------------------------ nothing | nothing | nothing | nothing kprobe | kprobe | tracer | kprobe jumpprobe | jumpprobe | tracer | jumpprobe printk | printk | tracer | kprobe tracer | tracer | tracer | tracer
Therefore, selecting the "kprobe" configuration option would still let people instrument the hardest paths while having mostly dynamic probes.
Mathieu
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