Re: Tracing configuration review

[Chase Douglas <chase.douglas@canonical.com>]

On Tue, 2010-05-25 at 15:46 -0400, Steven Rostedt wrote:
> On Tue, 2010-05-25 at 15:31 -0400, Chase Douglas wrote:
> > Hi all,
> > 
> > I'm going through our Ubuntu kernel configuration for our next release
> > to ensure we have all the trace options enabled that may be useful. I
> > have a few questions about what tracer options we should have enabled.
> > 
> > Our guiding principle in regards to these options is: if an option can
> > be turned on and has no performance impact unless explicitly enabled on
> > the kernel command line or at runtime, we are happy to enable it.
> > Secondarily, we don't want to enable options that are headed for
> > deprecation.
> > 
> > The following options are what I am looking to set for our x86
> > configurations. I've only included those that I am not 100% sure of.
> > Comments are what I could gather from documentation and Kconfig, but
> > they may not be accurate:
> > 
> > # CONFIG_IRQSOFF_TRACER is not set (performance impact by default)
> 
> Correct, keep that off.
> 
> > # CONFIG_SYSPROF_TRACER is not set (don't know much about this)
> 
> Neither do I ;-)
> 
> 
> > # CONFIG_SCHED_TRACER is not set (headed for deprecation?)
> 
> Although it is headed for deprecation, I think it still gets set by
> other tracers, since it has the code to initiate the comm reader.
> 
> > CONFIG_FTRACE_SYSCALLS=y (no performance impact by default)
> 
> Correct
> 
> > CONFIG_BOOT_TRACER=y (no performance impact by default)
> 
> But this tracer is pretty useless. It gives no more information than
> debug_initcalls.
> 
> > CONFIG_KSYM_TRACER=y (no performance impact by default)
> 
> Yep
> 
> > # CONFIG_STACK_TRACER is not set (Kconfig says N if unsure)
> 
> I would set this if you already have the function tracer. It gives no
> more overhead than that, and it is very useful.
> 
> > # CONFIG_KMEMTRACE is not set (Kconfig says N if unsure)
> 
> Don't know.
> 
> > CONFIG_WORKQUEUE_TRACER=y (no performance impact by default)
> > 
> > Lastly, what options are safe for performance when
> > HAVE_DYNAMIC_FTRACE=n, like on our ARM kernels. It is not clear to me
> > through what's in Documentation/trace/* and the Kconfig entries what
> > options could cause a performance decrease due to the inability to
> > dynamically enable and disable tracing at runtime.
> 
> HAVE_DYNAMIC_FTRACE affects the function tracer. If you do not have
> that, then do not enable FUNCTION_TRACER or anything that depends on it.
> 
> Also note, FUNCTION_TRACER depends on FRAME_POINTERS. Your millage may
> vary with that. If you already have FRAME_POINTERS on, and the arch
> supports DYNAMIC_FTRACE, then its fine to have FUNCTION_TRACER and all
> those that are built on top (STACK_TRACER, FUNCTION_GRAPH, etc)

(I would have snipped much of the above, but since I've added CC for the
Ubuntu list I wanted to leave it as is for this first reply)

After enabling KSYM_TRACER, I was presented with PROFILE_KSYM_TRACER.
This is also "Say N if unsure," so I could use some guidance on whether
we should turn it on as well.

Thanks,

-- Chase