Btrfs: track transid for delayed ref flushing
diff mbox

Message ID 1460410660-17430-1-git-send-email-jbacik@fb.com
State Superseded
Headers show

Commit Message

Josef Bacik April 11, 2016, 9:37 p.m. UTC
Using the offwakecputime bpf script I noticed most of our time was spent waiting
on the delayed ref throttling.  This is what is supposed to happen, but
sometimes the transaction can commit and then we're waiting for throttling that
doesn't matter anymore.  So change this stuff to be a little smarter by tracking
the transid we were in when we initiated the throttling.  If the transaction we
get is different then we can just bail out.  This resulted in a 50% speedup in
my fs_mark test, and reduced the amount of time spent throttling by 60 seconds
over the entire run (which is about 30 minutes).  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
---
 fs/btrfs/ctree.h       |  2 +-
 fs/btrfs/extent-tree.c | 15 ++++++++++++---
 fs/btrfs/inode.c       |  1 +
 fs/btrfs/transaction.c |  3 ++-
 4 files changed, 16 insertions(+), 5 deletions(-)

Comments

Liu Bo April 12, 2016, 5:43 p.m. UTC | #1
On Mon, Apr 11, 2016 at 05:37:40PM -0400, Josef Bacik wrote:
> Using the offwakecputime bpf script I noticed most of our time was spent waiting
> on the delayed ref throttling.  This is what is supposed to happen, but
> sometimes the transaction can commit and then we're waiting for throttling that
> doesn't matter anymore.  So change this stuff to be a little smarter by tracking
> the transid we were in when we initiated the throttling.  If the transaction we
> get is different then we can just bail out.  This resulted in a 50% speedup in
> my fs_mark test, and reduced the amount of time spent throttling by 60 seconds
> over the entire run (which is about 30 minutes).  Thanks,

Does the bpf script show where it's waiting on?  delayed_refs spinlock?

Maybe we can make it even smarter.

In __btrfs_end_transaction(), the only case it won't wait for running delayed refs
is when trans is JOIN_NOLOCK or ATTACH and "must_run_delayed_refs = 2".

In other cases, even we queue a work into helper worker to do async
delayed refs processing, __btrfs_end_transaction() still waits there.

Since it's a 50% speedup, it looks like at least 50% of __btrfs_end_transaction()
are waiting for other trans's queued delayed refs, can we do the check
a little bit earlier, in btrfs_async_run_delayed_refs()?

> 
> Signed-off-by: Josef Bacik <jbacik@fb.com>
> ---
>  fs/btrfs/ctree.h       |  2 +-
>  fs/btrfs/extent-tree.c | 15 ++++++++++++---
>  fs/btrfs/inode.c       |  1 +
>  fs/btrfs/transaction.c |  3 ++-
>  4 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 55a24c5..4222936 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3505,7 +3505,7 @@ void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
>  int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
>  			   struct btrfs_root *root, unsigned long count);
>  int btrfs_async_run_delayed_refs(struct btrfs_root *root,
> -				 unsigned long count, int wait);
> +				 unsigned long count, u64 transid, int wait);
>  int btrfs_lookup_data_extent(struct btrfs_root *root, u64 start, u64 len);
>  int btrfs_lookup_extent_info(struct btrfs_trans_handle *trans,
>  			     struct btrfs_root *root, u64 bytenr,
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 4b5a517..f23f426 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2839,6 +2839,7 @@ int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
>  
>  struct async_delayed_refs {
>  	struct btrfs_root *root;
> +	u64 transid;
>  	int count;
>  	int error;
>  	int sync;
> @@ -2854,9 +2855,16 @@ static void delayed_ref_async_start(struct btrfs_work *work)
>  
>  	async = container_of(work, struct async_delayed_refs, work);
>  
> -	trans = btrfs_join_transaction(async->root);
> +	trans = btrfs_attach_transaction(async->root);
>  	if (IS_ERR(trans)) {
> -		async->error = PTR_ERR(trans);
> +		if (PTR_ERR(trans) != -ENOENT)
> +			async->error = PTR_ERR(trans);
> +		goto done;
> +	}
> +
> +	/* Don't bother flushing if we got into a different transaction */
> +	if (trans->transid != async->transid) {
> +		btrfs_end_transaction(trans, async->root);

Interesting, btrfs_end_transaction will also issue a work to do 
delayed_ref_async_start, and it doesn't wait.

Thanks,

-liubo

>  		goto done;
>  	}
>  
> @@ -2880,7 +2888,7 @@ done:
>  }
>  
>  int btrfs_async_run_delayed_refs(struct btrfs_root *root,
> -				 unsigned long count, int wait)
> +				 unsigned long count, u64 transid, int wait)
>  {
>  	struct async_delayed_refs *async;
>  	int ret;
> @@ -2892,6 +2900,7 @@ int btrfs_async_run_delayed_refs(struct btrfs_root *root,
>  	async->root = root->fs_info->tree_root;
>  	async->count = count;
>  	async->error = 0;
> +	async->transid = transid;
>  	if (wait)
>  		async->sync = 1;
>  	else
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 723e4bb..e6dd4cc 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -4534,6 +4534,7 @@ delete:
>  			BUG_ON(ret);
>  			if (btrfs_should_throttle_delayed_refs(trans, root))
>  				btrfs_async_run_delayed_refs(root,
> +							     trans->transid,
>  					trans->delayed_ref_updates * 2, 0);
>  			if (be_nice) {
>  				if (truncate_space_check(trans, root,
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 43885e5..7c7671d 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -817,6 +817,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>  {
>  	struct btrfs_transaction *cur_trans = trans->transaction;
>  	struct btrfs_fs_info *info = root->fs_info;
> +	u64 transid = trans->transid;
>  	unsigned long cur = trans->delayed_ref_updates;
>  	int lock = (trans->type != TRANS_JOIN_NOLOCK);
>  	int err = 0;
> @@ -904,7 +905,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>  
>  	kmem_cache_free(btrfs_trans_handle_cachep, trans);
>  	if (must_run_delayed_refs) {
> -		btrfs_async_run_delayed_refs(root, cur,
> +		btrfs_async_run_delayed_refs(root, cur, transid,
>  					     must_run_delayed_refs == 1);
>  	}
>  	return err;
> -- 
> 2.5.0
> 
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Josef Bacik April 12, 2016, 5:55 p.m. UTC | #2
On 04/12/2016 01:43 PM, Liu Bo wrote:
> On Mon, Apr 11, 2016 at 05:37:40PM -0400, Josef Bacik wrote:
>> Using the offwakecputime bpf script I noticed most of our time was spent waiting
>> on the delayed ref throttling.  This is what is supposed to happen, but
>> sometimes the transaction can commit and then we're waiting for throttling that
>> doesn't matter anymore.  So change this stuff to be a little smarter by tracking
>> the transid we were in when we initiated the throttling.  If the transaction we
>> get is different then we can just bail out.  This resulted in a 50% speedup in
>> my fs_mark test, and reduced the amount of time spent throttling by 60 seconds
>> over the entire run (which is about 30 minutes).  Thanks,
>
> Does the bpf script show where it's waiting on?  delayed_refs spinlock?

We are waiting on the wait_for_completion() in 
btrfs_async_run_delayed_refs.  The script only catches where we're in 
TASK_UNINTERRUPTIBLE for longer than 100ms.

>
> Maybe we can make it even smarter.
>
> In __btrfs_end_transaction(), the only case it won't wait for running delayed refs
> is when trans is JOIN_NOLOCK or ATTACH and "must_run_delayed_refs = 2".
>
> In other cases, even we queue a work into helper worker to do async
> delayed refs processing, __btrfs_end_transaction() still waits there.
>
> Since it's a 50% speedup, it looks like at least 50% of __btrfs_end_transaction()
> are waiting for other trans's queued delayed refs, can we do the check
> a little bit earlier, in btrfs_async_run_delayed_refs()?

We'd have to start another transaction, we don't want to have to do 
that.  What I want to do later is have the flushing stuff running all 
the time, so we notice way sooner if we end up with a bunch of people 
all trying to throttle at once.  So we drop below the throttle watermark 
and everybody wakes up, instead of everybody does their work no matter 
what and then wakes up.  But this was quick and easy and I've got other 
stuff to do so this is what I posted ;).  Thanks,

Josef

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Patch
diff mbox

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 55a24c5..4222936 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3505,7 +3505,7 @@  void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
 int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 			   struct btrfs_root *root, unsigned long count);
 int btrfs_async_run_delayed_refs(struct btrfs_root *root,
-				 unsigned long count, int wait);
+				 unsigned long count, u64 transid, int wait);
 int btrfs_lookup_data_extent(struct btrfs_root *root, u64 start, u64 len);
 int btrfs_lookup_extent_info(struct btrfs_trans_handle *trans,
 			     struct btrfs_root *root, u64 bytenr,
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 4b5a517..f23f426 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2839,6 +2839,7 @@  int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
 
 struct async_delayed_refs {
 	struct btrfs_root *root;
+	u64 transid;
 	int count;
 	int error;
 	int sync;
@@ -2854,9 +2855,16 @@  static void delayed_ref_async_start(struct btrfs_work *work)
 
 	async = container_of(work, struct async_delayed_refs, work);
 
-	trans = btrfs_join_transaction(async->root);
+	trans = btrfs_attach_transaction(async->root);
 	if (IS_ERR(trans)) {
-		async->error = PTR_ERR(trans);
+		if (PTR_ERR(trans) != -ENOENT)
+			async->error = PTR_ERR(trans);
+		goto done;
+	}
+
+	/* Don't bother flushing if we got into a different transaction */
+	if (trans->transid != async->transid) {
+		btrfs_end_transaction(trans, async->root);
 		goto done;
 	}
 
@@ -2880,7 +2888,7 @@  done:
 }
 
 int btrfs_async_run_delayed_refs(struct btrfs_root *root,
-				 unsigned long count, int wait)
+				 unsigned long count, u64 transid, int wait)
 {
 	struct async_delayed_refs *async;
 	int ret;
@@ -2892,6 +2900,7 @@  int btrfs_async_run_delayed_refs(struct btrfs_root *root,
 	async->root = root->fs_info->tree_root;
 	async->count = count;
 	async->error = 0;
+	async->transid = transid;
 	if (wait)
 		async->sync = 1;
 	else
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 723e4bb..e6dd4cc 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4534,6 +4534,7 @@  delete:
 			BUG_ON(ret);
 			if (btrfs_should_throttle_delayed_refs(trans, root))
 				btrfs_async_run_delayed_refs(root,
+							     trans->transid,
 					trans->delayed_ref_updates * 2, 0);
 			if (be_nice) {
 				if (truncate_space_check(trans, root,
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 43885e5..7c7671d 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -817,6 +817,7 @@  static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 {
 	struct btrfs_transaction *cur_trans = trans->transaction;
 	struct btrfs_fs_info *info = root->fs_info;
+	u64 transid = trans->transid;
 	unsigned long cur = trans->delayed_ref_updates;
 	int lock = (trans->type != TRANS_JOIN_NOLOCK);
 	int err = 0;
@@ -904,7 +905,7 @@  static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 
 	kmem_cache_free(btrfs_trans_handle_cachep, trans);
 	if (must_run_delayed_refs) {
-		btrfs_async_run_delayed_refs(root, cur,
+		btrfs_async_run_delayed_refs(root, cur, transid,
 					     must_run_delayed_refs == 1);
 	}
 	return err;