Date: Thu, 4 Apr 2024 20:58:12 +0200
From: David Sterba <dsterba@...e.cz>
To: Sasha Levin <sashal@...nel.org>
Cc: David Sterba <dsterba@...e.cz>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, Filipe Manana <fdmanana@...e.com>,
	Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
	clm@...com, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 6.8 59/68] btrfs: preallocate temporary extent
 buffer for inode logging when needed

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@...e.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@...icpanda.com>
> >> Signed-off-by: Filipe Manana <fdmanana@...e.com>
> >> Reviewed-by: David Sterba <dsterba@...e.com>
> >> Signed-off-by: David Sterba <dsterba@...e.com>
> >> Signed-off-by: Sasha Levin <sashal@...nel.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.

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