[PATCH] setlocalversion: Add workaround for "git describe" performance issue

[Josh Poimboeuf <jpoimboe@kernel.org>]

If HEAD isn't associated with an annotated tag, a bug (or feature?) in
"git describe --match" causes it to search every commit in the entire
repository looking for additional match candidates.  Instead of it
taking a fraction of a second, it adds 10-15 seconds to the beginning of
every kernel build.

Fix it by adding an additional dummy match which is slightly further
away from the most recent one, along with setting the max candidate
count to 1 (not 2, apparently another bug).

Before:

  $ git checkout c1e939a21eb1
  $ time make kernel/fork.o -s

  real	0m12.403s
  user	0m11.591s
  sys	0m0.967s

After:

  $ time make kernel/fork.o -s

  real	0m1.119s
  user	0m0.658s
  sys	0m0.659s

Link: https://lore.kernel.org/git/20241030044322.b5n3ji2n6gaeo5u6@treble.attlocal.net/
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 scripts/setlocalversion | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/scripts/setlocalversion b/scripts/setlocalversion
index 38b96c6797f4..bb8c0bcb7368 100755
--- a/scripts/setlocalversion
+++ b/scripts/setlocalversion
@@ -57,6 +57,8 @@ scm_version()
 		return
 	fi
 
+	githack=" --match=v6.11 --candidates=1"
+
 	# mainline kernel:  6.2.0-rc5  ->  v6.2-rc5
 	# stable kernel:    6.1.7      ->  v6.1.7
 	version_tag=v$(echo "${KERNELVERSION}" | sed -E 's/^([0-9]+\.[0-9]+)\.0(.*)$/\1\2/')
@@ -67,7 +69,7 @@ scm_version()
 	tag=${file_localversion#-}
 	desc=
 	if [ -n "${tag}" ]; then
-		desc=$(git describe --match=$tag 2>/dev/null)
+		desc=$(git describe --match=$tag $githack 2>/dev/null)
 	fi
 
 	# Otherwise, if a localversion* file exists, and the tag
@@ -76,13 +78,13 @@ scm_version()
 	# it. This is e.g. the case in linux-rt.
 	if [ -z "${desc}" ] && [ -n "${file_localversion}" ]; then
 		tag="${version_tag}${file_localversion}"
-		desc=$(git describe --match=$tag 2>/dev/null)
+		desc=$(git describe --match=$tag $githack 2>/dev/null)
 	fi
 
 	# Otherwise, default to the annotated tag derived from KERNELVERSION.
 	if [ -z "${desc}" ]; then
 		tag="${version_tag}"
-		desc=$(git describe --match=$tag 2>/dev/null)
+		desc=$(git describe --match=$tag $githack 2>/dev/null)
 	fi
 
 	# If we are at the tagged commit, we ignore it because the version is
-- 
2.47.0