pack-bitmap: remove trace2 region from hot path

The trace2 region around the call to lazy_bitmap_for_commit() in
bitmap_for_commit() was added in 28cd730 (pack-bitmap: prepare to
read lookup table extension, 2022-08-14). While adding trace2 regions is
typically helpful for tracking performance, this method is called
possibly thousands of times as a commit walk explores commit history
looking for a matching bitmap. When trace2 output is enabled, this
region is emitted many times and performance is throttled by that
output.

For now, remove these regions entirely.

This is a critical path, and it would be valuable to measure that the
time spent in bitmap_for_commit() does not increase when using the
commit lookup table. The best way to do that would be to use a mechanism
that sums the time spent in a region and reports a single value at the
end of the process. This technique was introduced but not merged by [1]
so maybe this example presents some justification to revisit that
approach.

[1] https://lore.kernel.org/git/[email protected]/

To help with the 'git blame' output in this region, add a comment that
warns against adding a trace2 region. Delete a test from t5310 that used
that trace output to check that this lookup optimization was activated.
To create this kind of test again in the future, the stopwatch traces
mentioned earlier could be used as a signal that we activated this code
path.

Helpedy-by: Junio C Hamano <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

pack-bitmap.c

@@ -830,10 +830,9 @@ struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git,
 		if (!bitmap_git->table_lookup)
 			return NULL;
 
-		trace2_region_enter("pack-bitmap", "reading_lookup_table", the_repository);
+		/* this is a fairly hot codepath - no trace2_region please */
 		/* NEEDSWORK: cache misses aren't recorded */
 		bitmap = lazy_bitmap_for_commit(bitmap_git, commit);
-		trace2_region_leave("pack-bitmap", "reading_lookup_table", the_repository);
 		if (!bitmap)
 			return NULL;
 		return lookup_stored_bitmap(bitmap);

t/t5310-pack-bitmaps.sh

@@ -455,13 +455,6 @@ test_expect_success 'verify writing bitmap lookup table when enabled' '
 	grep "\"label\":\"writing_lookup_table\"" trace2
 '
 
-test_expect_success 'lookup table is actually used to traverse objects' '
-	git repack -adb &&
-	GIT_TRACE2_EVENT="$(pwd)/trace3" \
-		git rev-list --use-bitmap-index --count --all &&
-	grep "\"label\":\"reading_lookup_table\"" trace3
-'
-
 test_expect_success 'truncated bitmap fails gracefully (lookup table)' '
 	test_config pack.writebitmaphashcache false &&
 	git repack -adb &&