On Tue, Apr 02, 2024 at 03:35:18PM +0200, David Sterba wrote:
On Fri, Mar 29, 2024 at 08:25:55AM -0400, Sasha Levin wrote:
From: Filipe Manana fdmanana@suse.com
[ Upstream commit e383e158ed1b6abc2d2d3e6736d77a46393f80fa ]
When logging an inode and we require to copy items from subvolume leaves to the log tree, we clone each subvolume leaf and than use that clone to copy items to the log tree. This is required to avoid possible deadlocks as stated in commit 796787c978ef ("btrfs: do not modify log tree while holding a leaf from fs tree locked").
The cloning requires allocating an extent buffer (struct extent_buffer) and then allocating pages (folios) to attach to the extent buffer. This may be slow in case we are under memory pressure, and since we are doing the cloning while holding a read lock on a subvolume leaf, it means we can be blocking other operations on that leaf for significant periods of time, which can increase latency on operations like creating other files, renaming files, etc. Similarly because we're under a log transaction, we may also cause extra delay on other tasks doing an fsync, because syncing the log requires waiting for tasks that joined a log transaction to exit the transaction.
So to improve this, for any inode logging operation that needs to copy items from a subvolume leaf ("full sync" or "copy everything" bit set in the inode), preallocate a dummy extent buffer before locking any extent buffer from the subvolume tree, and even before joining a log transaction, add it to the log context and then use it when we need to copy items from a subvolume leaf to the log tree. This avoids making other operations get extra latency when waiting to lock a subvolume leaf that is used during inode logging and we are under heavy memory pressure.
The following test script with bonnie++ was used to test this:
$ cat test.sh #!/bin/bash
DEV=/dev/sdh MNT=/mnt/sdh MOUNT_OPTIONS="-o ssd"
MEMTOTAL_BYTES=`free -b | grep Mem: | awk '{ print $2 }'` NR_DIRECTORIES=20 NR_FILES=20480 DATASET_SIZE=$((MEMTOTAL_BYTES * 2 / 1048576)) DIRECTORY_SIZE=$((MEMTOTAL_BYTES * 2 / NR_FILES)) NR_FILES=$((NR_FILES / 1024))
echo "performance" | \ tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
umount $DEV &> /dev/null mkfs.btrfs -f $MKFS_OPTIONS $DEV mount $MOUNT_OPTIONS $DEV $MNT
bonnie++ -u root -d $MNT \ -n $NR_FILES:$DIRECTORY_SIZE:$DIRECTORY_SIZE:$NR_DIRECTORIES \ -r 0 -s $DATASET_SIZE -b
umount $MNT
The results of this test on a 8G VM running a non-debug kernel (Debian's default kernel config), were the following.
Before this change:
Version 2.00a ------Sequential Output------ --Sequential Input- --Random- -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks-- Name:Size etc /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP debian0 7501M 376k 99 1.4g 96 117m 14 1510k 99 2.5g 95 +++++ +++ Latency 35068us 24976us 2944ms 30725us 71770us 26152us Version 2.00a ------Sequential Create------ --------Random Create-------- debian0 -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete-- files:max:min /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP 20:384100:384100/20 20480 32 20480 58 20480 48 20480 39 20480 56 20480 61 Latency 411ms 11914us 119ms 617ms 10296us 110ms
After this change:
Version 2.00a ------Sequential Output------ --Sequential Input- --Random- -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks-- Name:Size etc /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP debian0 7501M 375k 99 1.4g 97 117m 14 1546k 99 2.3g 98 +++++ +++ Latency 35975us 20945us 2144ms 10297us 2217us 6004us Version 2.00a ------Sequential Create------ --------Random Create-------- debian0 -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete-- files:max:min /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP 20:384100:384100/20 20480 35 20480 58 20480 48 20480 40 20480 57 20480 59 Latency 320ms 11237us 77779us 518ms 6470us 86389us
Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Filipe Manana fdmanana@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Sasha Levin sashal@kernel.org
This is a performance improvement, how does this qualify for stable? I read only about notable perfromance fixes but this is not one.
No objection to dropping it. Description of the commit states that it fixes blocking for "significant amount of time".