Performance impact of CONFIG_FUNCTION_TRACER

[Sascha Hauer]

Hi,

I ran some lmbench subtests on a ARMv7 machine (NXP i.MX6q) with and
without CONFIG_FUNCTION_TRACER enabled (with CONFIG_DYNAMIC_FTRACE
enabled and no tracing active), see below. The Kconfig help text of this
option reads as:

> If it's runtime disabled (the bootup default), then the overhead of
> the instructions is very small and not measurable even in
> micro-benchmarks.

In my tests the overhead is small, but it surely exists and is
measurable at least on ARMv7 machines. Is this expected? Should the help
text be rephrased a little less optimistic?

lat_fs
======

CONFIG_FUNCTION_TRACER=y
------------------------

  _ file size
 /        _ repetitions
 |       /         _ created files per second
 |       |        /        _ deleted files per second
 |       |        |       /
0k      205     38325   70264
1k      123     21949   41495
4k      119     21632   42365
10k     84      14408   29439

CONFIG_FUNCTION_TRACER=n
------------------------

0k      198     39317   76241
1k      125     22397   42420
4k      125     22038   44144
10k     84      14965   30032

./scripts/do_ctx
================

Measures context switch time (in us?)

CONFIG_FUNCTION_TRACER=y
------------------------
"size=0k ovr=4.60
2 16.57
4 15.75
8 16.99
16 16.93
24 18.79
32 20.11
64 22.20
96 21.59

CONFIG_FUNCTION_TRACER=n
------------------------
"size=0k ovr=4.88
2 15.26
4 17.29
8 18.25
16 18.40
24 22.19
32 20.85
64 23.32
96 23.11
 
lat_pipe
========

I think it measures the time between sending and receiving data
through a pipe.

CONFIG_FUNCTION_TRACER=y
------------------------
Pipe latency: 44.7039 microseconds

CONFIG_FUNCTION_TRACER=n
------------------------
Pipe latency: 41.9876 microseconds

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