Messages in this thread | | | Date | Tue, 25 May 2010 22:20:52 +0200 | From | Frederic Weisbecker <> | Subject | Re: Tracing configuration review |
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On Tue, May 25, 2010 at 03:58:18PM -0400, Chase Douglas wrote: > 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.
KSYM_TRACER traces memory accesses (using breakpoints) and display each of these in the trace/trace_pipe files. PROFILE_KSYM_TRACER produces basic statistics on top of these traces.
But both are deprecated really, the perf tools interface is much better for this job.
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