[PATCH 0/4] btrfs: a few fsync related minor improvements and a cleanup

[PATCH 0/4] btrfs: a few fsync related minor improvements and a cleanup

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

The following patches remove some unnecessary code and bring a couple minor
performance improvements in the fsync path. They are independent of each other,
but are grouped in the same pathset just because they relate around the same
code. The last patch has some performance results in its changelog.

Filipe Manana (4):
  btrfs: remove racy and unnecessary inode transaction update when using
    no-holes
  btrfs: avoid unnecessary log mutex contention when syncing log
  btrfs: remove unnecessary list head initialization when syncing log
  btrfs: avoid unnecessary lock and leaf splits when updating inode in
    the log

 fs/btrfs/inode.c    | 12 ++++------
 fs/btrfs/tree-log.c | 56 ++++++++++++++++++++++++++++++++++++---------
 2 files changed, 50 insertions(+), 18 deletions(-)

-- 
2.30.2

[PATCH 1/4] btrfs: remove racy and unnecessary inode transaction update when using no-holes

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

When using the NO_HOLES feature and expanding the size of an inode, we
update the inode's last_trans, last_sub_trans and last_log_commit fields
at maybe_insert_hole() so that a fsync does know that the inode needs to
be logged (by making sure that btrfs_inode_in_log() returns false). This
happens for expanding truncate operations, buffered writes, direct IO
writes and when cloning extents to an offset greater than the inode's
i_size.

However the way we do it is racy, because in between setting the inode's
last_sub_trans and last_log_commit fields, the log transaction ID that was
assigned to last_sub_trans might be committed before we read the root's
last_log_commit and assign that value to last_log_commit. If that happens
it would make a future call to btrfs_inode_in_log() return true. This is
a race that should be extremely unlikely to be hit in practice, and it is
the same that was described by commit bc0939fcfab0d7 ("btrfs: fix race
between marking inode needs to be logged and log syncing").

The fix would simply be to set last_log_commit to the value we assigned
to last_sub_trans minus 1, like it was done in that commit. However
updating these two fields plus the last_trans field is pointless here
because all the callers of btrfs_cont_expand() (which is the only
caller of maybe_insert_hole()) always call btrfs_set_inode_last_trans()
or btrfs_update_inode() after calling btrfs_cont_expand(). Calling either
btrfs_set_inode_last_trans() or btrfs_update_inode() guarantees that the
next fsync will log the inode, as it makes btrfs_inode_in_log() return
false.

So just remove the code that explicitly sets the inode's last_trans,
last_sub_trans and last_log_commit fields.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/inode.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index e6eb20987351..5798c6191908 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4999,15 +4999,13 @@ static int maybe_insert_hole(struct btrfs_root *root, struct btrfs_inode *inode,
 	int ret;
 
 	/*
-	 * Still need to make sure the inode looks like it's been updated so
-	 * that any holes get logged if we fsync.
+	 * If NO_HOLES is enabled, we don't need to do anything.
+	 * Later, up in the call chain, either btrfs_set_inode_last_sub_trans()
+	 * or btrfs_update_inode() will be called, which guarantee that the next
+	 * fsync will know this inode was changed and needs to be logged.
 	 */
-	if (btrfs_fs_incompat(fs_info, NO_HOLES)) {
-		inode->last_trans = fs_info->generation;
-		inode->last_sub_trans = root->log_transid;
-		inode->last_log_commit = root->last_log_commit;
+	if (btrfs_fs_incompat(fs_info, NO_HOLES))
 		return 0;
-	}
 
 	/*
 	 * 1 - for the one we're dropping
-- 
2.30.2

[PATCH 2/4] btrfs: avoid unnecessary log mutex contention when syncing log

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

When syncing the log we acquire the root's log mutex just to update the
root's last_log_commit. This is unnecessary because:

1) At this point there can only be one task updating this value, which is
   the task committing the current log transaction. Any task that enters
   btrfs_sync_log() has to wait for the previous log transaction to commit
   and wait for the current log transaction to commit if someone else
   already started it (in this case it never reaches to the point of
   updating last_log_commit, as that is done by the comitting task);

2) All readers of the root's last_log_commit don't acquire the root's
   log mutex. This is to avoid blocking the readers, potentially for too
   long and because getting a stale value of last_log_commit does not
   cause any functional problem, in the worst case getting a stale value
   results in logging an inode unnecessarily. Plus it's actually very
   rare to get a stale value that results in unnecessarily logging the
   inode.

So in order to avoid unnecessary contention on the root's log mutex,
which is used for several different purposes, like starting/joining a
log transaction and starting writeback of a log transaction, stop
acquiring the log mutex for updating the root's last_log_commit.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/tree-log.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 9fd0348be7f5..90fb5a2fc60b 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3328,10 +3328,16 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 		goto out_wake_log_root;
 	}
 
-	mutex_lock(&root->log_mutex);
-	if (root->last_log_commit < log_transid)
-		root->last_log_commit = log_transid;
-	mutex_unlock(&root->log_mutex);
+	/*
+	 * We know there can only be one task here, since we have not yet set
+	 * root->log_commit[index1] to 0 and any task attempting to sync the
+	 * log must wait for the previous log transaction to commit if it's
+	 * still in progress or wait for the current log transaction commit if
+	 * someone else already started it. We use <= and not < because the
+	 * first log transaction has an ID of 0.
+	 */
+	ASSERT(root->last_log_commit <= log_transid);
+	root->last_log_commit = log_transid;
 
 out_wake_log_root:
 	mutex_lock(&log_root_tree->log_mutex);
-- 
2.30.2

[PATCH 3/4] btrfs: remove unnecessary list head initialization when syncing log

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

One of the last steps of syncing the log is to remove all log contextes
from the root's list of contextes, done at btrfs_remove_all_log_ctxs().
There we iterate over all the contextes in the list and delete each one
from the list, and after that we call INIT_LIST_HEAD() on the list. That
is unnecessary since at that point the list is empty.

So just remove the INIT_LIST_HEAD() call. It's not needed, increases code
size (bloat-o-meter reported a delta of -122 for btrfs_sync_log() after
this change) and increases two critical sections delimited by log mutexes.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/tree-log.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 90fb5a2fc60b..63f48715135c 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3039,8 +3039,6 @@ static inline void btrfs_remove_all_log_ctxs(struct btrfs_root *root,
 		list_del_init(&ctx->list);
 		ctx->log_ret = error;
 	}
-
-	INIT_LIST_HEAD(&root->log_ctxs[index]);
 }
 
 /*
-- 
2.30.2

Re: [PATCH 3/4] btrfs: remove unnecessary list head initialization when syncing log

[Nikolay Borisov]

On 20.07.21 г. 18:03, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> One of the last steps of syncing the log is to remove all log contextes
> from the root's list of contextes, done at btrfs_remove_all_log_ctxs().
> There we iterate over all the contextes in the list and delete each one
> from the list, and after that we call INIT_LIST_HEAD() on the list. That
> is unnecessary since at that point the list is empty.
> 
> So just remove the INIT_LIST_HEAD() call. It's not needed, increases code
nit: I assume you mean decreases code size
> size (bloat-o-meter reported a delta of -122 for btrfs_sync_log() after
> this change) and increases two critical sections delimited by log mutexes.

nit: Here you also mean decreases two critsecs

> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>  fs/btrfs/tree-log.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> index 90fb5a2fc60b..63f48715135c 100644
> --- a/fs/btrfs/tree-log.c
> +++ b/fs/btrfs/tree-log.c
> @@ -3039,8 +3039,6 @@ static inline void btrfs_remove_all_log_ctxs(struct btrfs_root *root,
>  		list_del_init(&ctx->list);
>  		ctx->log_ret = error;
>  	}
> -
> -	INIT_LIST_HEAD(&root->log_ctxs[index]);
>  }
>  
>  /*
> 

Re: [PATCH 3/4] btrfs: remove unnecessary list head initialization when syncing log

[Filipe Manana]

On Fri, Jul 23, 2021 at 7:20 AM Nikolay Borisov <nborisov@suse.com> wrote:
>
>
>
> On 20.07.21 г. 18:03, fdmanana@kernel.org wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> >
> > One of the last steps of syncing the log is to remove all log contextes
> > from the root's list of contextes, done at btrfs_remove_all_log_ctxs().
> > There we iterate over all the contextes in the list and delete each one
> > from the list, and after that we call INIT_LIST_HEAD() on the list. That
> > is unnecessary since at that point the list is empty.
> >
> > So just remove the INIT_LIST_HEAD() call. It's not needed, increases code
> nit: I assume you mean decreases code size
> > size (bloat-o-meter reported a delta of -122 for btrfs_sync_log() after
> > this change) and increases two critical sections delimited by log mutexes.
>
> nit: Here you also mean decreases two critsecs

No, in both cases I meant "increases".
The goal of the sentence is to list the negative consequences of
having the call, that's why the sentence starts with "It's not needed,
".

Thanks.

>
> >
> > Signed-off-by: Filipe Manana <fdmanana@suse.com>
> > ---
> >  fs/btrfs/tree-log.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> > index 90fb5a2fc60b..63f48715135c 100644
> > --- a/fs/btrfs/tree-log.c
> > +++ b/fs/btrfs/tree-log.c
> > @@ -3039,8 +3039,6 @@ static inline void btrfs_remove_all_log_ctxs(struct btrfs_root *root,
> >               list_del_init(&ctx->list);
> >               ctx->log_ret = error;
> >       }
> > -
> > -     INIT_LIST_HEAD(&root->log_ctxs[index]);
> >  }
> >
> >  /*
> >

[PATCH 4/4] btrfs: avoid unnecessary lock and leaf splits when updating inode in the log

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

During a fast fsync, if we have already fsynced the file before and in the
current transaction, we can make the inode item update more efficient and
avoid acquiring a write lock on the leaf's parent.

To update the inode item we are always using btrfs_insert_empty_item() to
get a path pointing to the inode item, which calls btrfs_search_slot()
with an "ins_len" argument of 'sizeof(struct btrfs_inode_item) +
sizeof(struct btrfs_item)', and that always results in the search taking
a write lock on the level 1 node that is the parent of the leaf that
contains the inode item. This adds unnecessary lock contention on log
trees when we have multiple fsyncs in parallel against inodes in the same
subvolume, which has a very significant impact due to the fact that log
trees are short lived and their height very rarely goes beyond level 2.

Also, by using btrfs_insert_empty_item() when we need to update the inode
item, we also end up splitting the leaf of the existing inode item when
the leaf has an amount of free space smaller than the size of an inode
item.

Improve this by using btrfs_seach_slot(), with a 0 "ins_len" argument,
when we know the inode item already exists in the log. This avoids these
two inefficiencies.

The following script, using fio, was used to perform the tests:

  $ cat fio-test.sh
  #!/bin/bash

  DEV=/dev/nvme0n1
  MNT=/mnt/nvme0n1
  MOUNT_OPTIONS="-o ssd"
  MKFS_OPTIONS="-d single -m single"

  if [ $# -ne 4 ]; then
    echo "Use $0 NUM_JOBS FILE_SIZE FSYNC_FREQ BLOCK_SIZE"
    exit 1
  fi

  NUM_JOBS=$1
  FILE_SIZE=$2
  FSYNC_FREQ=$3
  BLOCK_SIZE=$4

  cat <<EOF > /tmp/fio-job.ini
  [writers]
  rw=randwrite
  fsync=$FSYNC_FREQ
  fallocate=none
  group_reporting=1
  direct=0
  bs=$BLOCK_SIZE
  ioengine=sync
  size=$FILE_SIZE
  directory=$MNT
  numjobs=$NUM_JOBS
  EOF

  echo "performance" | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

  echo
  echo "Using config:"
  echo
  cat /tmp/fio-job.ini
  echo
  echo "mount options: $MOUNT_OPTIONS"
  echo

  umount $MNT &> /dev/null
  mkfs.btrfs -f $MKFS_OPTIONS $DEV
  mount $MOUNT_OPTIONS $DEV $MNT

  fio /tmp/fio-job.ini
  umount $MNT

The tests were done on a physical machine, with 12 cores, 64G of RAM,
using a NVMEe device and using a non-debug kernel config (the default one
from Debian). The summary line from fio is provided below for each test
run.

With 8 jobs, file size 256M, fsync frequency of 4 and a block size of 4K:

Before: WRITE: bw=28.3MiB/s (29.7MB/s), 28.3MiB/s-28.3MiB/s (29.7MB/s-29.7MB/s), io=2048MiB (2147MB), run=72297-72297msec
After:  WRITE: bw=28.7MiB/s (30.1MB/s), 28.7MiB/s-28.7MiB/s (30.1MB/s-30.1MB/s), io=2048MiB (2147MB), run=71411-71411msec

+1.4% throughput, -1.2% runtime

With 16 jobs, file size 256M, fsync frequency of 4 and a block size of 4K:

Before: WRITE: bw=40.0MiB/s (42.0MB/s), 40.0MiB/s-40.0MiB/s (42.0MB/s-42.0MB/s), io=4096MiB (4295MB), run=99980-99980msec
After:  WRITE: bw=40.9MiB/s (42.9MB/s), 40.9MiB/s-40.9MiB/s (42.9MB/s-42.9MB/s), io=4096MiB (4295MB), run=97933-97933msec

+2.2% throughput, -2.1% runtime

The changes are small but it's possible to be better on faster hardware as
in the test machine used disk utilization was pretty much 100% during the
whole time the tests were running (observed with 'iostat -xz 1').

The tests also included the previous patch with the subject of:
"btrfs: avoid unnecessary log mutex contention when syncing log".
So they compared a branch without that patch and without this patch versus
a branch with these two patches applied.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/tree-log.c | 40 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 63f48715135c..2fa8815f7b3b 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3972,14 +3972,41 @@ static void fill_inode_item(struct btrfs_trans_handle *trans,
 
 static int log_inode_item(struct btrfs_trans_handle *trans,
 			  struct btrfs_root *log, struct btrfs_path *path,
-			  struct btrfs_inode *inode)
+			  struct btrfs_inode *inode, bool inode_item_dropped)
 {
 	struct btrfs_inode_item *inode_item;
 	int ret;
 
-	ret = btrfs_insert_empty_item(trans, log, path,
-				      &inode->location, sizeof(*inode_item));
-	if (ret && ret != -EEXIST)
+	/*
+	 * If we are doing a fast fsync and the inode was logged before in the
+	 * current transaction, then we know the inode was previously logged and
+	 * it exists in the log tree. For performance reasons, in this case use
+	 * btrfs_search_slot() directly with ins_len set to 0 so that we never
+	 * attempt a write lock on the leaf's parent, which adds unnecessary lock
+	 * contention in case there are concurrent fsyncs for other inodes of the
+	 * same subvolume. Using btrfs_insert_empty_item() when the inode item
+	 * already exists can also result in unnecessarily splitting a leaf.
+	 */
+	if (!inode_item_dropped && inode->logged_trans == trans->transid) {
+		ret = btrfs_search_slot(trans, log, &inode->location, path, 0, 1);
+		ASSERT(ret <= 0);
+		if (ret > 0)
+			ret = -ENOENT;
+	} else {
+		/*
+		 * This means it is the first fsync in the current transaction,
+		 * so the inode item is not in the log and we need to insert it.
+		 * We can never get -EEXIST because we are only called for a fast
+		 * fsync and in case an inode eviction happens after the inode was
+		 * logged before in the current transaction, when we load again
+		 * the inode, we set BTRFS_INODE_NEEDS_FULL_SYNC on its runtime
+		 * flags and set ->logged_trans to 0.
+		 */
+		ret = btrfs_insert_empty_item(trans, log, path, &inode->location,
+					      sizeof(*inode_item));
+		ASSERT(ret != -EEXIST);
+	}
+	if (ret)
 		return ret;
 	inode_item = btrfs_item_ptr(path->nodes[0], path->slots[0],
 				    struct btrfs_inode_item);
@@ -5303,6 +5330,7 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 	bool need_log_inode_item = true;
 	bool xattrs_logged = false;
 	bool recursive_logging = false;
+	bool inode_item_dropped = true;
 
 	path = btrfs_alloc_path();
 	if (!path)
@@ -5437,6 +5465,7 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 		} else {
 			if (inode_only == LOG_INODE_ALL)
 				fast_search = true;
+			inode_item_dropped = false;
 			goto log_extents;
 		}
 
@@ -5470,7 +5499,8 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 	btrfs_release_path(path);
 	btrfs_release_path(dst_path);
 	if (need_log_inode_item) {
-		err = log_inode_item(trans, log, dst_path, inode);
+		err = log_inode_item(trans, log, dst_path, inode,
+				     inode_item_dropped);
 		if (err)
 			goto out_unlock;
 		/*
-- 
2.30.2

Re: [PATCH 0/4] btrfs: a few fsync related minor improvements and a cleanup

[Josef Bacik]

On 7/20/21 11:03 AM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> The following patches remove some unnecessary code and bring a couple minor
> performance improvements in the fsync path. They are independent of each other,
> but are grouped in the same pathset just because they relate around the same
> code. The last patch has some performance results in its changelog.
> 
> Filipe Manana (4):
>    btrfs: remove racy and unnecessary inode transaction update when using
>      no-holes
>    btrfs: avoid unnecessary log mutex contention when syncing log
>    btrfs: remove unnecessary list head initialization when syncing log
>    btrfs: avoid unnecessary lock and leaf splits when updating inode in
>      the log
> 
>   fs/btrfs/inode.c    | 12 ++++------
>   fs/btrfs/tree-log.c | 56 ++++++++++++++++++++++++++++++++++++---------
>   2 files changed, 50 insertions(+), 18 deletions(-)
> 

You can add

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

to this series, thanks,

Josef

Re: [PATCH 0/4] btrfs: a few fsync related minor improvements and a cleanup

[David Sterba]

On Tue, Jul 20, 2021 at 04:03:39PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> The following patches remove some unnecessary code and bring a couple minor
> performance improvements in the fsync path. They are independent of each other,
> but are grouped in the same pathset just because they relate around the same
> code. The last patch has some performance results in its changelog.
> 
> Filipe Manana (4):
>   btrfs: remove racy and unnecessary inode transaction update when using
>     no-holes
>   btrfs: avoid unnecessary log mutex contention when syncing log
>   btrfs: remove unnecessary list head initialization when syncing log
>   btrfs: avoid unnecessary lock and leaf splits when updating inode in
>     the log

Added to misc-next, thanks.