[PATCH v2 0/6] A variety of lock contention fixes

[PATCH v2 0/6] A variety of lock contention fixes

[Josef Bacik]

v1->v2:
- Fixed the log messages that Nikolay pointed out.
- Added Nikolay's reviewed by for the first patch.
- Removed the unneeded mb for flushing.

--- Original email ---
Hello,

I've been running some stress tests recently in order to try and reproduce some
problems I've tripped over in relocation.  Most of this series is a reposting of
patches I wrote when debugging related issues for Zygo that got lost.  I've
updated one of them to make the lock contention even better, making it so I have
to ramp up my stress test loops because it now finishes way too fast.  Thanks,

Josef

Josef Bacik (6):
  btrfs: do not block on deleted bgs mutex in the cleaner
  btrfs: only let one thread pre-flush delayed refs in commit
  btrfs: delayed refs pre-flushing should only run the heads we have
  btrfs: only run delayed refs once before committing
  btrfs: stop running all delayed refs during snapshot
  btrfs: run delayed refs less often in commit_cowonly_roots

 fs/btrfs/block-group.c | 11 +++++--
 fs/btrfs/delayed-ref.h | 12 +++----
 fs/btrfs/extent-tree.c |  2 +-
 fs/btrfs/transaction.c | 74 ++++++++++++++++--------------------------
 4 files changed, 43 insertions(+), 56 deletions(-)

-- 
2.24.1

[PATCH v2 1/6] btrfs: do not block on deleted bgs mutex in the cleaner

[Josef Bacik]

While running some stress tests I started getting hung task messages.
This is because the delete unused bg's code has to take the
delete_unused_bgs_mutex to do it's work, which is taken by balance to
make sure we don't delete block groups while we're balancing.

The problem is a balance can take a while, and so we were getting hung
task warnings.  We don't need to block and run these things, and the
cleaner is needed to do other work, so trylock on this mutex and just
bail if we can't acquire it right away.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/block-group.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 3ba6f3839d39..2b5f91b35032 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1333,6 +1333,13 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
 	if (!test_bit(BTRFS_FS_OPEN, &fs_info->flags))
 		return;
 
+	/*
+	 * Long running balances can keep us blocked here for eternity, so
+	 * simply skip deletion if we're unable to get the mutex.
+	 */
+	if (!mutex_trylock(&fs_info->delete_unused_bgs_mutex))
+		return;
+
 	spin_lock(&fs_info->unused_bgs_lock);
 	while (!list_empty(&fs_info->unused_bgs)) {
 		int trimming;
@@ -1352,8 +1359,6 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
 
 		btrfs_discard_cancel_work(&fs_info->discard_ctl, block_group);
 
-		mutex_lock(&fs_info->delete_unused_bgs_mutex);
-
 		/* Don't want to race with allocators so take the groups_sem */
 		down_write(&space_info->groups_sem);
 
@@ -1499,11 +1504,11 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
 end_trans:
 		btrfs_end_transaction(trans);
 next:
-		mutex_unlock(&fs_info->delete_unused_bgs_mutex);
 		btrfs_put_block_group(block_group);
 		spin_lock(&fs_info->unused_bgs_lock);
 	}
 	spin_unlock(&fs_info->unused_bgs_lock);
+	mutex_unlock(&fs_info->delete_unused_bgs_mutex);
 	return;
 
 flip_async:
-- 
2.24.1

[PATCH v2 2/6] btrfs: only let one thread pre-flush delayed refs in commit

[Josef Bacik]

I've been running a stress test that runs 20 workers in their own
subvolume, which are running an fsstress instance with 4 threads per
worker, which is 80 total fsstress threads.  In addition to this I'm
running balance in the background as well as creating and deleting
snapshots.  This test takes around 12 hours to run normally, going
slower and slower as the test goes on.

The reason for this is because fsstress is running fsync sometimes, and
because we're messing with block groups we often fall through to
btrfs_commit_transaction, so will often have 20-30 threads all calling
btrfs_commit_transaction at the same time.

These all get stuck contending on the extent tree while they try to run
delayed refs during the initial part of the commit.

This is suboptimal, really because the extent tree is a single point of
failure we only want one thread acting on that tree at once to reduce
lock contention.  Fix this by making the flushing mechanism a bit
operation, to make it easy to use test_and_set_bit() in order to make
sure only one task does this initial flush.

Once we're into the transaction commit we only have one thread doing
delayed ref running, it's just this initial pre-flush that is
problematic.  With this patch my stress test takes around 90 minutes to
run, instead of 12 hours.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/delayed-ref.h | 12 ++++++------
 fs/btrfs/transaction.c | 33 ++++++++++++++++-----------------
 2 files changed, 22 insertions(+), 23 deletions(-)

diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
index 1c977e6d45dc..6e414785b56f 100644
--- a/fs/btrfs/delayed-ref.h
+++ b/fs/btrfs/delayed-ref.h
@@ -135,6 +135,11 @@ struct btrfs_delayed_data_ref {
 	u64 offset;
 };
 
+enum btrfs_delayed_ref_flags {
+	/* Used to indicate that we are flushing delayed refs for the commit. */
+	BTRFS_DELAYED_REFS_FLUSHING,
+};
+
 struct btrfs_delayed_ref_root {
 	/* head ref rbtree */
 	struct rb_root_cached href_root;
@@ -158,12 +163,7 @@ struct btrfs_delayed_ref_root {
 
 	u64 pending_csums;
 
-	/*
-	 * set when the tree is flushing before a transaction commit,
-	 * used by the throttling code to decide if new updates need
-	 * to be run right away
-	 */
-	int flushing;
+	unsigned long flags;
 
 	u64 run_delayed_start;
 
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 52ada47aff50..f73f3c7aeda2 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -870,9 +870,9 @@ int btrfs_should_end_transaction(struct btrfs_trans_handle *trans)
 {
 	struct btrfs_transaction *cur_trans = trans->transaction;
 
-	smp_mb();
 	if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
-	    cur_trans->delayed_refs.flushing)
+	    test_bit(BTRFS_DELAYED_REFS_FLUSHING,
+		     &cur_trans->delayed_refs.flags))
 		return 1;
 
 	return should_end_transaction(trans);
@@ -2042,23 +2042,22 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 	btrfs_trans_release_metadata(trans);
 	trans->block_rsv = NULL;
 
-	/* make a pass through all the delayed refs we have so far
-	 * any runnings procs may add more while we are here
-	 */
-	ret = btrfs_run_delayed_refs(trans, 0);
-	if (ret) {
-		btrfs_end_transaction(trans);
-		return ret;
-	}
-
-	cur_trans = trans->transaction;
-
 	/*
-	 * set the flushing flag so procs in this transaction have to
-	 * start sending their work down.
+	 * We only want one transaction commit doing the flushing so we do not
+	 * waste a bunch of time on lock contention on the extent root node.
 	 */
-	cur_trans->delayed_refs.flushing = 1;
-	smp_wmb();
+	if (!test_and_set_bit(BTRFS_DELAYED_REFS_FLUSHING,
+			      &cur_trans->delayed_refs.flags)) {
+		/*
+		 * make a pass through all the delayed refs we have so far
+		 * any runnings procs may add more while we are here
+		 */
+		ret = btrfs_run_delayed_refs(trans, 0);
+		if (ret) {
+			btrfs_end_transaction(trans);
+			return ret;
+		}
+	}
 
 	btrfs_create_pending_block_groups(trans);
 
-- 
2.24.1

Re: [PATCH v2 2/6] btrfs: only let one thread pre-flush delayed refs in commit

[Nikolay Borisov]

On 16.10.20 г. 18:52 ч., Josef Bacik wrote:
> I've been running a stress test that runs 20 workers in their own
> subvolume, which are running an fsstress instance with 4 threads per
> worker, which is 80 total fsstress threads.  In addition to this I'm
> running balance in the background as well as creating and deleting
> snapshots.  This test takes around 12 hours to run normally, going
> slower and slower as the test goes on.
> 
> The reason for this is because fsstress is running fsync sometimes, and
> because we're messing with block groups we often fall through to
> btrfs_commit_transaction, so will often have 20-30 threads all calling
> btrfs_commit_transaction at the same time.
> 
> These all get stuck contending on the extent tree while they try to run
> delayed refs during the initial part of the commit.
> 
> This is suboptimal, really because the extent tree is a single point of
> failure we only want one thread acting on that tree at once to reduce
> lock contention.  Fix this by making the flushing mechanism a bit
> operation, to make it easy to use test_and_set_bit() in order to make
> sure only one task does this initial flush.
> 
> Once we're into the transaction commit we only have one thread doing
> delayed ref running, it's just this initial pre-flush that is
> problematic.  With this patch my stress test takes around 90 minutes to
> run, instead of 12 hours.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

[PATCH v2 3/6] btrfs: delayed refs pre-flushing should only run the heads we have

[Josef Bacik]

Previously our delayed ref running used the total number of items as the
items to run.  However we changed that to number of heads to run with
the delayed_refs_rsv, as generally we want to run all of the operations
for one bytenr.

But with btrfs_run_delayed_refs(trans, 0) we set our count to 2x the
number of items that we have.  This is generally fine, but if we have
some operation generation loads of delayed refs while we're doing this
pre-flushing in the transaction commit, we'll just spin forever doing
delayed refs.

Fix this to simply pick the number of delayed refs we currently have,
that way we do not end up doing a lot of extra work that's being
generated in other threads.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/extent-tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 5fd60b13f4f8..a7f0a1480cd9 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2180,7 +2180,7 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 
 	delayed_refs = &trans->transaction->delayed_refs;
 	if (count == 0)
-		count = atomic_read(&delayed_refs->num_entries) * 2;
+		count = delayed_refs->num_heads_ready;
 
 again:
 #ifdef SCRAMBLE_DELAYED_REFS
-- 
2.24.1

Re: [PATCH v2 3/6] btrfs: delayed refs pre-flushing should only run the heads we have

[Nikolay Borisov]

On 16.10.20 г. 18:52 ч., Josef Bacik wrote:
> Previously our delayed ref running used the total number of items as the
> items to run.  However we changed that to number of heads to run with
> the delayed_refs_rsv, as generally we want to run all of the operations
> for one bytenr.
> 
> But with btrfs_run_delayed_refs(trans, 0) we set our count to 2x the
> number of items that we have.  This is generally fine, but if we have
> some operation generation loads of delayed refs while we're doing this
> pre-flushing in the transaction commit, we'll just spin forever doing
> delayed refs.
> 
> Fix this to simply pick the number of delayed refs we currently have,
> that way we do not end up doing a lot of extra work that's being
> generated in other threads.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> Reviewed-by: Nikolay Borisov <nborisov@suse.com>

Turns out this patch regresses on generic/371 so refrain from merging
for the time being.

<snip>

[PATCH v2 4/6] btrfs: only run delayed refs once before committing

[Josef Bacik]

We try to pre-flush the delayed refs when committing, because we want to
do as little work as possible in the critical section of the transaction
commit.

However doing this twice can lead to very long transaction commit delays
as other threads are allowed to continue to generate more delayed refs,
which potentially delays the commit by multiple minutes in very extreme
cases.

So simply stick to one pre-flush, and then continue the rest of the
transaction commit.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/transaction.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index f73f3c7aeda2..1b6fef512121 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -2061,12 +2061,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 
 	btrfs_create_pending_block_groups(trans);
 
-	ret = btrfs_run_delayed_refs(trans, 0);
-	if (ret) {
-		btrfs_end_transaction(trans);
-		return ret;
-	}
-
 	if (!test_bit(BTRFS_TRANS_DIRTY_BG_RUN, &cur_trans->flags)) {
 		int run_it = 0;
 
-- 
2.24.1

[PATCH v2 5/6] btrfs: stop running all delayed refs during snapshot

[Josef Bacik]

This was added a very long time ago to work around issues with delayed
refs and with qgroups.  Both of these issues have since been properly
fixed, so all this does is cause a lot of lock contention with anybody
else who is running delayed refs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/transaction.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 1b6fef512121..93006da039a3 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1650,12 +1650,6 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 		goto fail;
 	}
 
-	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
-	if (ret) {
-		btrfs_abort_transaction(trans, ret);
-		goto fail;
-	}
-
 	/*
 	 * Do special qgroup accounting for snapshot, as we do some qgroup
 	 * snapshot hack to do fast snapshot.
@@ -1703,12 +1697,6 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 		}
 	}
 
-	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
-	if (ret) {
-		btrfs_abort_transaction(trans, ret);
-		goto fail;
-	}
-
 fail:
 	pending->error = ret;
 dir_item_existed:
-- 
2.24.1

[PATCH v2 6/6] btrfs: run delayed refs less often in commit_cowonly_roots

[Josef Bacik]

We love running delayed refs in commit_cowonly_roots, but it is a bit
excessive.  I was seeing cases of running 3 or 4 refs a few times in a
row during this time.  Instead simply update all of the roots first,
then run delayed refs, then handle the empty block groups case, and then
if we have any more dirty roots do the whole thing again.  This allows
us to be much more efficient with our delayed ref running, as we can
batch a few more operations at once.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/transaction.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 93006da039a3..cefdd0dc4c20 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1188,10 +1188,6 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
 	btrfs_tree_unlock(eb);
 	free_extent_buffer(eb);
 
-	if (ret)
-		return ret;
-
-	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
 	if (ret)
 		return ret;
 
@@ -1209,10 +1205,6 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
 	if (ret)
 		return ret;
 
-	/* run_qgroups might have added some more refs */
-	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
-	if (ret)
-		return ret;
 again:
 	while (!list_empty(&fs_info->dirty_cowonly_roots)) {
 		struct btrfs_root *root;
@@ -1227,15 +1219,24 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
 		ret = update_cowonly_root(trans, root);
 		if (ret)
 			return ret;
-		ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
-		if (ret)
-			return ret;
 	}
 
+	/* Now flush any delayed refs generated by updating all of the roots. */
+	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
+	if (ret)
+		return ret;
+
 	while (!list_empty(dirty_bgs) || !list_empty(io_bgs)) {
 		ret = btrfs_write_dirty_block_groups(trans);
 		if (ret)
 			return ret;
+
+		/*
+		 * We're writing the dirty block groups, which could generate
+		 * delayed refs, which could generate more dirty block groups,
+		 * so we want to keep this flushing in this loop to make sure
+		 * everything gets run.
+		 */
 		ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
 		if (ret)
 			return ret;
-- 
2.24.1