btrfs: Test if btrfs will commit too many transactions for balance

Kernel commit 64403612b73a ("btrfs: rework
btrfs_check_space_for_delayed_refs") is introducing a regression for
btrfs balance performance.

Since that commit will cause btrfs to commit too many transactions
for nothing during balance/relocation, it will slow balance
dramatically even we only need to relocate several megabytes.

This test case will catch the problem by using super block
generation as failure criteria.

For small chunk relocated, we will commit 6 transactions for each
block group, and the test case should only have 2 block groups, it
should only commit 12 transactions.

This test case will use 120 as the threshold to detect the failure.

And in my test environment, with kernel fix btrfs committed 14
transactions. While without the fix btrfs committed 209
transactions.

So the test case should be enough to detect the regression, while still
keep the runtime small enough for failure.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/btrfs/181

@@ -0,0 +1,101 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2019 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test 181
+#
+# Test if btrfs will commit too many transactions for nothing and cause
+# performance regression during balance.
+#
+# This bug is going to be fixed by a patch for kernel title
+# "btrfs: don't end the transaction for delayed refs in throttle"
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs btrfs
+_supported_os Linux
+_require_scratch
+_require_btrfs_command inspect-internal dump-super
+
+_scratch_mkfs > /dev/null
+
+_scratch_mount
+
+nr_files=1024
+
+get_super_gen()
+{
+	local ret=$($BTRFS_UTIL_PROG inspect dump-super "$SCRATCH_DEV" |\
+		grep ^generation | awk '{print $2}')
+	if [ -z $ret ]; then
+		_fail "failed to get super block generation"
+	fi
+	echo "$ret"
+}
+
+$BTRFS_UTIL_PROG subvolume create "$SCRATCH_MNT/subvol" > /dev/null
+
+# Create some small files to take up enough metadata reserved space
+for ((i = 0; i < $nr_files; i++)) do
+	_pwrite_byte 0xcd 0 1K "$SCRATCH_MNT/subvol/file_$i" > /dev/null
+done
+
+# Commit the fs so we can get a stable super generation
+sync
+
+before_gen=$(get_super_gen)
+
+$BTRFS_UTIL_PROG balance start -m "$SCRATCH_MNT" > /dev/null
+
+after_gen=$(get_super_gen)
+
+# Since the fs is pretty small, we should have only 1 small metadata chunk and
+# one tiny system chunk.
+# Relocating such small chunks only needs 6 commits for each, thus 12 commits for
+# 2 chunks.
+# Here we use 10x the theoretic value as threshold.
+theoretic_gen=$(( 6 * 2 ))
+threshold_gen=$(( 10 * $theoretic_gen ))
+if [ $(( $after_gen - $before_gen )) -gt 120 ]; then
+	echo "balance committed too many transactions"
+	echo "super generation before balance: ${before_gen}"
+	echo "super generation after balance:  ${after_gen}"
+	echo "super generation difference:     $((after_gen - before_gen))"
+	echo "theoretic generation difference: ${theoretic_gen}"
+	echo "threshold generation difference: ${threshold_gen}"
+fi
+
+echo "super generation before balance: ${before_gen}" >> $seqres.full
+echo "super generation after balance:  ${after_gen}" >> $seqres.full
+echo "super generation difference:     $((after_gen - before_gen))" >> $seqres.full
+echo "theoretic generation difference: ${theoretic_gen}" >> $seqres.full
+echo "threshold generation difference: ${threshold_gen}" >> $seqres.full
+
+# success, all done
+echo "Silence is golden"
+
+status=0
+exit

tests/btrfs/181.out

@@ -0,0 +1,2 @@
+QA output created by 181
+Silence is golden

tests/btrfs/group

@@ -183,3 +183,4 @@
 178 auto quick send
 179 auto qgroup dangerous
 180 auto quick qgroup limit
+181 auto quick balance