linux-kernel - Re: [patch] trace: Add user-space event tracing/injection

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Date:	Wed, 17 Nov 2010 13:30:55 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [patch] trace: Add user-space event tracing/injection


* Pekka Enberg <penberg@...nel.org> wrote:

> >  $ trace report
> >  #
> >  # trace events of 'sleep 1':
> >  #
> >         testit/ 6006 ( 0.002 ms):<"Hello World!">
> >         testit/ 6006 ( 0.002 ms):<"Hello World!">
> 
> Wow! This looks really nice!
> 
> What does the duration in milliseconds mean there? For things like
> GC and JIT, I want something like:
> 
> void gc(void)
> {
>         prctl(PR_TASK_PERF_USER_TRACE_START, ...)
> 
>         collect();
> 
>         prctl(PR_TASK_PERF_USER_TRACE_END, ...)
> }
> 
> So that it's clear from the tracing output that the VM was busy
> doing GC for n milliseconds. Barring background JIT'ing and
> pauseless GC, I'd also be interested in showing how much time the VM
> was actually _blocking_ the running application (which can happen
> with signals too, btw, for things like accessing data that's lazily
> initialized).

We can add two events: user_event_entry/user_event_exit - or we could use the string 
to differentiate, and start it with:

  "entry: ..."
  "exit: ..."

And then the event timestamps (which are absolute and are available) could be used 
to calculate the duration of this period.

'trace' could even be taught to treat such entry:/exit: strings in a special way, so 
that you dont have to write Jato specific trace decoding bits?

Thanks,

	Ingo
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