Re: [PATCH 3/3] btrfs: replace cleaner_delayed_iput_mutex with a waitqueue

[David Sterba <dsterba@suse.cz>]

On Fri, Jan 11, 2019 at 10:21:03AM -0500, Josef Bacik wrote:
> The throttle path doesn't take cleaner_delayed_iput_mutex, which means
> we could think we're done flushing iputs in the data space reservation
> path when we could have a throttler doing an iput.  There's no real
> reason to serialize the delayed iput flushing, so instead of taking the
> cleaner_delayed_iput_mutex whenever we flush the delayed iputs just
> replace it with an atomic counter and a waitqueue.  This removes the
> short (or long depending on how big the inode is) window where we think
> there are no more pending iputs when there really are some.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/ctree.h       |  4 +++-
>  fs/btrfs/disk-io.c     |  5 ++---
>  fs/btrfs/extent-tree.c | 13 ++++++++-----
>  fs/btrfs/inode.c       | 21 +++++++++++++++++++++
>  4 files changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index dc56a4d940c3..20af5d6d81f1 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -915,7 +915,8 @@ struct btrfs_fs_info {
>  
>  	spinlock_t delayed_iput_lock;
>  	struct list_head delayed_iputs;
> -	struct mutex cleaner_delayed_iput_mutex;
> +	atomic_t nr_delayed_iputs;
> +	wait_queue_head_t delayed_iputs_wait;
>  
>  	/* this protects tree_mod_seq_list */
>  	spinlock_t tree_mod_seq_lock;
> @@ -3240,6 +3241,7 @@ int btrfs_orphan_cleanup(struct btrfs_root *root);
>  int btrfs_cont_expand(struct inode *inode, loff_t oldsize, loff_t size);
>  void btrfs_add_delayed_iput(struct inode *inode);
>  void btrfs_run_delayed_iputs(struct btrfs_fs_info *fs_info);
> +int btrfs_wait_on_delayed_iputs(struct btrfs_fs_info *fs_info);
>  int btrfs_prealloc_file_range(struct inode *inode, int mode,
>  			      u64 start, u64 num_bytes, u64 min_size,
>  			      loff_t actual_len, u64 *alloc_hint);
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index f40f6fdc1019..238e0113f2d3 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -1694,9 +1694,7 @@ static int cleaner_kthread(void *arg)
>  			goto sleep;
>  		}
>  
> -		mutex_lock(&fs_info->cleaner_delayed_iput_mutex);
>  		btrfs_run_delayed_iputs(fs_info);
> -		mutex_unlock(&fs_info->cleaner_delayed_iput_mutex);
>  
>  		again = btrfs_clean_one_deleted_snapshot(root);
>  		mutex_unlock(&fs_info->cleaner_mutex);
> @@ -2654,7 +2652,6 @@ int open_ctree(struct super_block *sb,
>  	mutex_init(&fs_info->delete_unused_bgs_mutex);
>  	mutex_init(&fs_info->reloc_mutex);
>  	mutex_init(&fs_info->delalloc_root_mutex);
> -	mutex_init(&fs_info->cleaner_delayed_iput_mutex);
>  	seqlock_init(&fs_info->profiles_lock);
>  
>  	INIT_LIST_HEAD(&fs_info->dirty_cowonly_roots);
> @@ -2676,6 +2673,7 @@ int open_ctree(struct super_block *sb,
>  	atomic_set(&fs_info->defrag_running, 0);
>  	atomic_set(&fs_info->qgroup_op_seq, 0);
>  	atomic_set(&fs_info->reada_works_cnt, 0);
> +	atomic_set(&fs_info->nr_delayed_iputs, 0);
>  	atomic64_set(&fs_info->tree_mod_seq, 0);
>  	fs_info->sb = sb;
>  	fs_info->max_inline = BTRFS_DEFAULT_MAX_INLINE;
> @@ -2753,6 +2751,7 @@ int open_ctree(struct super_block *sb,
>  	init_waitqueue_head(&fs_info->transaction_wait);
>  	init_waitqueue_head(&fs_info->transaction_blocked_wait);
>  	init_waitqueue_head(&fs_info->async_submit_wait);
> +	init_waitqueue_head(&fs_info->delayed_iputs_wait);
>  
>  	INIT_LIST_HEAD(&fs_info->pinned_chunks);
>  
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 0127d272cd2a..5b6c9fc227ff 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4280,10 +4280,14 @@ int btrfs_alloc_data_chunk_ondemand(struct btrfs_inode *inode, u64 bytes)
>  				/*
>  				 * The cleaner kthread might still be doing iput
>  				 * operations. Wait for it to finish so that
> -				 * more space is released.
> +				 * more space is released.  We don't need to
> +				 * explicitly run the delayed iputs here because
> +				 * the commit_transaction would have woken up
> +				 * the cleaner.
>  				 */
> -				mutex_lock(&fs_info->cleaner_delayed_iput_mutex);
> -				mutex_unlock(&fs_info->cleaner_delayed_iput_mutex);
> +				ret = btrfs_wait_on_delayed_iputs(fs_info);
> +				if (ret)
> +					return ret;
>  				goto again;
>  			} else {
>  				btrfs_end_transaction(trans);
> @@ -4958,9 +4962,8 @@ static void flush_space(struct btrfs_fs_info *fs_info,
>  		 * bunch of pinned space, so make sure we run the iputs before
>  		 * we do our pinned bytes check below.
>  		 */
> -		mutex_lock(&fs_info->cleaner_delayed_iput_mutex);
>  		btrfs_run_delayed_iputs(fs_info);
> -		mutex_unlock(&fs_info->cleaner_delayed_iput_mutex);
> +		btrfs_wait_on_delayed_iputs(fs_info);
>  
>  		ret = may_commit_transaction(fs_info, space_info);
>  		break;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 0b9f3e482cea..958e30c7c744 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -3260,6 +3260,7 @@ void btrfs_add_delayed_iput(struct inode *inode)
>  	if (atomic_add_unless(&inode->i_count, -1, 1))
>  		return;
>  
> +	atomic_inc(&fs_info->nr_delayed_iputs);
>  	spin_lock(&fs_info->delayed_iput_lock);
>  	ASSERT(list_empty(&binode->delayed_iput));
>  	list_add_tail(&binode->delayed_iput, &fs_info->delayed_iputs);
> @@ -3280,11 +3281,31 @@ void btrfs_run_delayed_iputs(struct btrfs_fs_info *fs_info)
>  		list_del_init(&inode->delayed_iput);
>  		spin_unlock(&fs_info->delayed_iput_lock);
>  		iput(&inode->vfs_inode);
> +		if (atomic_dec_and_test(&fs_info->nr_delayed_iputs))
> +			wake_up(&fs_info->delayed_iputs_wait);
>  		spin_lock(&fs_info->delayed_iput_lock);
>  	}
>  	spin_unlock(&fs_info->delayed_iput_lock);
>  }
>  
> +/**
> + * btrfs_wait_on_delayed_iputs - wait on the delayed iputs to be done running
> + * @fs_info - the fs_info for this fs
> + * @return - EINTR if we were killed, 0 if nothing's pending
> + *
> + * This will wait on any delayed iputs that are currently running with KILLABLE
> + * set.  Once they are all done running we will return, unless we are killed in
> + * which case we return EINTR.

I still wonder from which context the 'killable' property is used.

btrfs_alloc_data_chunk_ondemand is indirectly inside zero range and
fallocate, there it makes sense. With flush_space is less obvious, so
this should be explained in the changelog what's expected.

> + */
> +int btrfs_wait_on_delayed_iputs(struct btrfs_fs_info *fs_info)
> +{
> +	int ret = wait_event_killable(fs_info->delayed_iputs_wait,
> +			atomic_read(&fs_info->nr_delayed_iputs) == 0);
> +	if (ret)
> +		return -EINTR;
> +	return 0;
> +}
> +
>  /*
>   * This creates an orphan entry for the given inode in case something goes wrong
>   * in the middle of an unlink.
> -- 
> 2.14.3