linux-kernel - Re: Tracing Requirements (was: [RFC/Requirements/Design] h/w error reporting)

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Date:	Wed, 10 Nov 2010 23:49:44 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Luck, Tony" <tony.luck@...el.com>, linux-kernel@...r.kernel.org,
	ying.huang@...el.com, bp@...en8.de, tglx@...utronix.de,
	akpm@...ux-foundation.org, mchehab@...hat.com,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: Tracing Requirements (was: [RFC/Requirements/Design] h/w error
	reporting)

On Wed, Nov 10, 2010 at 04:54:19PM -0500, Steven Rostedt wrote:
> On Wed, 2010-11-10 at 22:30 +0100, Frederic Weisbecker wrote:
> > >     - 0 ns IRQ/SOFTIRQ handler duration side-effect
> > 
> > 
> > 
> > ditto.
> 
> If an interrupt (or softirq) preempts the recorded trace, then events
> that are recorded in that interrupt all get the same time as the event
> it preempted. Giving us the assumption that all events happened at once.
> 
> Again, this is just a side effect and the fix is trivial. But may
> require ABI breakage to do so.



Right, now I recall I discussed that with Mathieu lately.


 
> 
> > 
> > If we need/want to cure that, then we need an:
> > 
> > => ABI breakage
> > 
> > 
> > 
> > > - Event size limited to one page
> > 
> > 
> > 
> > Perf too needs more (userspace stack dumps).
> 
> That was actually a decision made by Linus. But is trivial to change. As
> there's nothing hard coded about the design that forces us to have page
> size sub buffers. I don't even think that it would require an ABI
> breakage, except I think my tools I wrote (incorrectly) assumed it.


Ok.



> 
> > 
> > 
> > 
> > > - Ftrace event headers are still too large
> > 
> > 
> > (described in the beginning)
> 
> Yep, they are large, but can be trimmed. This would require no abi
> breakage since the these headers are also described in the event
> formats. Thus changing the current tools should cope with the headers
> changing. In fact they were designed too since the lock-depth was known
> to be deprecated soon.


Hmm, in practice this is an ABI breakage as we have scripts that rely on
the common_pid field for example. We can fix this, but older tools won't
work with new kernels.



> > > - Handling of dynamically added instrumentation while trace is recorded is
> > >   inexistent.
> > 
> > 
> > 
> > 
> > I still don't understand this point
> 
> He's talking about tracing the tracepoints in a loaded module. We
> currently have no way to add them while a trace is happening. The trace
> formats do not exist and may not exist (if module is unloaded) when the
> trace ends.
> 
> But who really loads and then unloads a module during tracing. As pretty
> much all kernel developers cringe at the fact that modules get
> unloaded ;-)



Hehe :)

Anyway this can be expressed through an ABI extension,
using a kind of lazy tracepoint registration or so.

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