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

David Sterba
2024-04-04T19:05:38Z

On Tue, Apr 02, 2024 at 08:33:55PM -0400, Sasha Levin wrote:
> 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".

I see, that would make sense as keyword for stable backport, though it
applies under heavy memory pressure so not a regular workload where I'd
consider it for stable right away. A system under load will block on
many allocations, from that perspective the patch may not make any
difference.