Re: [RFC v3][PATCH 0/2] Make ftrace able to trace function return

[Ingo Molnar <mingo@elte.hu>]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> This patchset adds the ability for ftrace to trace the function even 
> on call time and on return time. So we can now measure the time of 
> execution of the most part of the functions inside the kernel with 
> ftrace.
> 
> The first patch bring the low level tools to add the support of 
> return tracing on X86-32. It is totally separated from the 
> traditional implementation of ftrace and doesn't support dynamic 
> ftrace at this time.
> 
> The second patch adds a tracer based on the ring-buffer which 
> measure the time of execution of the functions inside the kernel.

okay, i've created tip/tracing/function-return-tracer topic so that we 
can start testing this for real and do can start iterating it via 
smaller changes. I've also integrated it into tip/master, it's looking 
good so far.

i did a couple of cleanups straight away:

 1) i did a FTRACE_RETURN => FUNCTION_RET_TRACER rename, to move it in
    line with the function tracer

 2) i cleaned up the arch/x86/Kconfig impact:

      f1c4be5: tracing, x86: clean up FUNCTION_RET_TRACER Kconfig

the return-tracer builds and boots, and works fabulously:

 # cd /sys/debug/tracing/

 # cat available_tracers 
 return function sched_switch nop

 # echo return > current_tracer
 
 clocksource_read+0xd/0xf -> acpi_pm_read (1547 ns)
 getnstimeofday+0x3e/0xc8 -> clocksource_read (1951 ns)
 ktime_get_ts+0x25/0x49 -> getnstimeofday (2354 ns)
 ktime_get_ts+0x45/0x49 -> set_normalized_timespec (120 ns)

Small quirk, it does not seem possible to switch from ftrace to 
ftrace-ret:

 # echo ftrace > current_tracer 
 # echo return > current_tracer
 bash: echo: write error: Invalid argument

i have to go via the "nop" tracer for this to work. Steve: switching 
between tracer plugins should be seemless.

Question: have you thought about extending the return-tracer to 
dyn-function-tracer?

That's where it will really shine: the dftrace workflow is to 
typically enable a low number of functions - and there the 
return-tracing overhead does not show up nearly as much as in the 
static-function-tracing workflow.

Another suggestion: i think the "return" plugin name is confusing to 
users (it confused me when i first saw it listed in 
available_tracers).

So lets use "function_full" and "function" tracing perhaps? The "full" 
tracer is what traces both entry and exit points, and establishes full 
function-call timings/costs. The "function" tracer is more lightweight 
and traces function entry events.

	Ingo