linux-kernel - Re: [RFC] convert ftrace syscall tracer to TRACE

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Date:	Sat, 9 May 2009 10:37:37 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Baron <jbaron@...hat.com>, Tom Zanussi <tzanussi@...il.com>
Cc:	linux-kernel@...r.kernel.org, fweisbec@...il.com,
	laijs@...fujitsu.com, rostedt@...dmis.org, peterz@...radead.org,
	mathieu.desnoyers@...ymtl.ca, jiayingz@...gle.com,
	mbligh@...gle.com, roland@...hat.com, fche@...hat.com
Subject: Re: [RFC] convert ftrace syscall tracer to TRACE_EVENT()


* Jason Baron <jbaron@...hat.com> wrote:

> Hi,
> 
> I've been thinking about converting the current ftrace syscall 
> tracer to the TRACE_EVENT() macros. There are a few issues with 
> the current syscall tracer approach:
> 
> 1) It has to be enabled for all processes and all syscalls. By 
> moving to TRACE_EVENT(), it can be enabled/disabled per tracepoint 
> and can also make use of the generic tracing filters, such as 
> "trace all process for pid x"
> 
> 2) Other tracers can not tie into it, since its not tracepoint 
> based. TRACE_EVENT() fixes this.
> 
> 3) data formatting. The syscall tracer I don't believe understands 
> all the various types for output formatting. By moving to 
> TRACE_EVENT(), we can print out a more readible syscall trace.
> 
> 4) The ftrace syscall tracer needs a new arch specific code for 
> each architecture. By converting to TRACE_EVENT() we don't need 
> any architecutre specific code.
> 
> Other issues to consider:
> 
> * Maintainence. The current syscall tracer automatically picks up 
> new syscalls. The TRACE_EVENT() will be harder to initially set 
> up. But once its done, syscalls are obviously not added often. So 
> I don't think this will be too bad.
> 
> * Performance. The current syscall tracer adds a 
> 'test_thread_flag()' to syscall entry/exit. The TRACE_EVENT() 
> would add a per-syscall global to check. So they are going to have 
> different cache profiles...however, the tracepoint infrastructure 
> is hopefully moving to the 'immediate' value work, which will make 
> this more highly optimized.
> 
> I've also tested the patch shown below (which uses, 
> DECLARE_TRACE(), as a preliminary proof of concept), using 
> getpid() in a loop, and tbench, and saw very small performance 
> differences. Obviously we would have to do more extensive testing 
> before deciding.
> 
> Patch is pretty rough, but should give a rough sense of what the 
> DECLARE_TRACE() type patch might look like...

Yeah, i very much agree with the direction. (I've Cc:-ed Tom Zanussi 
who also has expressed interest in this.)

I'm not sure about the implementation as you've posted it though:

Firstly, it adds two new tracepoints to every system call. That is 
unnecessary - we already have the TIF flag based callbacks, and we 
can use the existing syscall attributes table to get to tracepoints 
- without slow down (or impacting) the fast path in any way.

Secondly, we should reuse the information we get in SYSCALL_DEFINE, 
to construct the TRACE_EVENT tracepoints directly - without having 
to list all syscalls again in a separate file.

	Ingo
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