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

[Sasha Levin <sashal@kernel.org>]

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