* Ingo Molnar ([email protected]) wrote:
>
> * Mathieu Desnoyers <[email protected]> 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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