Re: [PATCH AUTOSEL 6.8 59/68] btrfs: preallocate temporary extent buffer for inode logging when needed

Sasha Levin
2024-04-03T00:33:55Z

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".

--
Thanks,
Sasha