There's another source of overhead in the perf version string 
generator:

   git update-index -q --refresh

... which will iterate the whole checked out tree. This can be 
pretty slow on NFS volumes, but takes some time even with local 
SSD disks and a fully cached kernel tree:

 $ perf stat --null --repeat 3 --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):

       0.306999221 seconds time elapsed                                          ( +-  0.56% )

So remove the .dirty differentiator as well - it adds little 
information because locally patched git trees are common, but 
seldom are the perf tools modified.

So a lot of version strings are reported as 'dirty' while in 
fact they are pristine perf builds. For example 99% of my perf 
builds are not patched but the kernel tree is slightly patched, 
which adds the .dirty tag.

Eliminating that tag speeds up version generation by another 
order of magnitude:

 $ perf stat --null --repeat 3 --sync --pre "rm -f PERF-VERSION-FILE" util/PERF-VERSION-GEN 
 PERF_VERSION = 3.7.rc3.g4b0bd3
 PERF_VERSION = 3.7.rc3.g4b0bd3
 PERF_VERSION = 3.7.rc3.g4b0bd3

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

       0.021270923 seconds time elapsed                                          ( +-  1.94% )

(Also clean up some of the comments around this code.)

Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 tools/perf/util/PERF-VERSION-GEN | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/tools/perf/util/PERF-VERSION-GEN b/tools/perf/util/PERF-VERSION-GEN
index c774b89..6fb1cc8 100755
--- a/tools/perf/util/PERF-VERSION-GEN
+++ b/tools/perf/util/PERF-VERSION-GEN

@@ -9,17 +9,12 @@ GVF=${OUTPUT}PERF-VERSION-FILE
 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
+# otherwise try to get the version from the kernel Makefile
+#
 if test -d ../../.git -o -f ../../.git &&
-	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]*)
-		git update-index -q --refresh
-		test -z "$(git diff-index --name-only HEAD --)" ||
-		VN="$VN-dirty" ;;
-	esac
+	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)
 then
 	VN=$(echo "$VN" | sed -e 's/-/./g');
 else