[PATCH] perf tools: Speed up the perf build time by simplifying the perf --version string generation

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> Btw., there's another thing that would be nice in addition to 
> simplifying the PERF-VERSION-GEN script: [...]

Here's a stab at that.

----------------->

Building perf is pretty slow on trees that have a lot of commits 
relative to the nearest Git tag. This slowness manifests itself 
during version string generation:

 $ perf stat --null --repeat 3 --sync --pre "rm -f PERF-VERSION-FILE" util/PERF-VERSION-GEN 
 PERF_VERSION = 3.7.rc3.1458.g5399b3b
 PERF_VERSION = 3.7.rc3.1458.g5399b3b
 PERF_VERSION = 3.7.rc3.1458.g5399b3b

 Performance counter stats for 'util/PERF-VERSION-GEN' (3 runs):

       2.857503976 seconds time elapsed                                          ( +-  0.22% )

The build can be even slower than that, when one over NFS 
volumes.

The reason for the slowness is that util/PERF-VERSION-GEN uses 
"git describe" to generate the string, which has to count the 
"number of commits distance" from the nearest tag - the ".1458." 
count in the output above. For that Git had to extract and 
decompress 1458 Git objects, which takes time and bandwidth.

But this "number of commits" value is mostly irrelevant in 
practice. We either want to know an approximate tag name, or we 
want to know the precise sha1.

So this patch simplifies the version string to:

 PERF_VERSION = 3.7.rc3.g5399b3b.dirty

which speeds up the version string generation script by an order 
of magnitude:

 $ perf stat --null --repeat 3 --sync --pre "rm -f PERF-VERSION-FILE" util/PERF-VERSION-GEN 
 PERF_VERSION = 3.7.rc3.g5399b3b.dirty
 PERF_VERSION = 3.7.rc3.g5399b3b.dirty
 PERF_VERSION = 3.7.rc3.g5399b3b.dirty

 Performance counter stats for 'util/PERF-VERSION-GEN' (3 runs):

       0.307633559 seconds time elapsed                                          ( +-  0.84% )

Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 tools/perf/util/PERF-VERSION-GEN | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/PERF-VERSION-GEN b/tools/perf/util/PERF-VERSION-GEN
index 95264f3..c774b89 100755
--- a/tools/perf/util/PERF-VERSION-GEN
+++ b/tools/perf/util/PERF-VERSION-GEN
@@ -12,7 +12,7 @@ LF='
 # First check if there is a .git to get the version from git describe
 # otherwise try to get the version from the kernel makefile
 if test -d ../../.git -o -f ../../.git &&
-	VN=$(git describe --match 'v[0-9].[0-9]*' --abbrev=4 HEAD 2>/dev/null) &&
+	VN=$(echo $(git tag --list "v[0-9].[0-9]*" | tail -1)"-g"$(git log -1 --abbrev=4 --pretty=format:"%h" HEAD) 2>/dev/null) &&
 	case "$VN" in
 	*$LF*) (exit 1) ;;
 	v[0-9]*)