Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Ingo Molnar (mingo@elte.hu) wrote:
> 
> * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> 
> > * Ingo Molnar (mingo@elte.hu) wrote:
> > > sorry, but i disagree. There _is_ a solution that is superior in every 
> > > aspect: kprobes + SystemTap. (or any other equivalent dynamic tracer)
> > > 
> > 
> > I am sorry to have to repeat myself, but this is not true for heavy 
> > loads.
> 
> djprobes?
> 

I am fully aware of djprobes limitations towards fully preemptible kernel (and
around branches instructions ? I don't remember if they solved this one). Oh,
yes, and if a trap happen to come at the wrong spot, then the thread gets
scheduled out... well, it cannot be applied everywhere, eh ?

> > > > At this point you've been rather uncompromising [...]
> > > 
> > > yes, i'm rather uncompromising when i sense attempts to push inferior 
> > > concepts into the core kernel _when_ a better concept exists here and 
> > > today. Especially if the concept being pushed adds more than 350 
> > > tracepoints that expose something to user-space that amounts to a 
> > > complex external API, which tracepoints we have little chance of ever 
> > > getting rid of under a static tracing concept.
> > > 
> > From an earlier email from Tim bird :
> > 
> > "I still think that this is off-topic for the patch posted.  I think 
> > we should debate the implementation of tracepoints/markers when 
> > someone posts a patch for some.  I think it's rather scurrilous to 
> > complain about code NOT submitted.  Ingo has even mis-characterized 
> > the not-submitted instrumentation patch, by saying it has 350 
> > tracepoints when it has no such thing.  I counted 58 for one 
> > architecture (with only 8 being arch-specific)."
> 
> i missed that (way too many mails in this thread).
> 
> Here is how i counted them:
> 
>  $ grep "\<trace_.*(" * | wc -l
>  359
> 

This count includes the inline trace functions definitions.

> some of those are not true tracepoints, but there's at least this many 
> of them:
> 
>  $ grep "\<trace_.*(" *instrumentation* | wc -l
>  235
> 

1 - This counts per architecture trace points. It quickly adds up considering
that we support ARM, MIPS, i386, powerpc, ppc and x86_64.
2 - It also counts some experimental trace points that I do not want to submit.
3 - Most of these are instrumentation of the traps handlers, which is
conceptually only one event.

> when judging kernel maintainance overhead, the sum of all patches 
> matters. And i considered all the other patches too (the ones that add 
> actual tracepoints) that will come after the currently offered ones, not 
> just the ones you submitted to lkml.
> 

I plan to rework the instrumentation patches before submitting them to LKML,
don't worry. I just hasn't been my focus until now. Too bad that you take those
as arguments.

Mathieu

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