Re: MARKER mechanism, try 2

[Mathieu Desnoyers]

* Ingo Molnar (mingo@xxxxxxx) wrote:
> 
> * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> 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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