tools/perf: Fix double/triple-build of the feature detection logic during 'make install' et al
Linus reported the following perf build system bug:
'Another annoyance during that make was that "make install" seems to
want to re-make the thing I just built. That's absolutely horrible, [...]'
The thing that got re-built were 'only' the (numerous) feature checks,
not the whole project - but still it was mighty annoying as the feature
checks took 9+ seconds even on reasonably fast boxes.
Even with the autodep patches where feature detection is much faster
it wastes resources, wastes screen real estate and confuses users if
we execute feature detection twice.
There were two sources for these unnecessary re-builds of the feature
checks:
- Unnecessary nested invocations of $(MAKE), apparently to be able
to do conditional compilation dependent on documentation tools
presence. Use straight dependencies instead, with no nesting.
- A direct invocation of $(MAKE) to rebuild the PERF-VERSION-FILE.
This is apparently done to be able to include it into the
Makefile:
-include $(OUTPUT)PERF-VERSION-FILE
but that's entirely pointless for two reasons: 1) the version file
gets regenerated by the initial build pass anyway, 2) including it
is futile, given its contents:
#define PERF_VERSION "3.12.rc3.g8510c7"
'make' will interpret that as a comment line...
So just remove this part of the doc-generation logic.
With these things fixed a 'make install' now rebuilds only what is needed.
A repeated 'make install' on an already built tree is super fast now,
it finishes in under 0.3 seconds:
#
# After the patch:
#
$ time make install
...
real 0m0.280s
user 0m0.162s
sys 0m0.054s
Prior all the autodep changes and prior this fix, a repeat 'make install'
took 24.1 seconds (!) on the same system:
#
# Before the patches:
#
$ time make install
...
real 0m24.109s
user 0m21.171s
sys 0m2.449s
Which almost entirely was caused by fixable build system fat.
We are now literally ~86 times faster.
A fresh rebuild and install now takes just 11.4 seconds:
#
# After the patch:
#
$ make clean
$ time make -j16 install
...
real 0m11.457s
user 1m43.411s
sys 0m7.610s
Without the patches it took 27.8 seconds:
#
# Before the patches:
#
$ make clean
$ time make -j16 install
...
real 0m27.801s
user 1m59.242s
sys 0m9.749s
So even in the complete rebuild case we are now ~2.5 times faster.
Reported-by: 
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-x4qjnxjGrgxpribq8sdakfTp@git.kernel.orgSigned-off-by: 
Ingo Molnar <mingo@kernel.org>
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