Re: [PATCH][v2] btrfs: run delayed iput at unlink time

[Nikolay Borisov <nborisov@suse.com>]

On 18.06.19 г. 17:59 ч., Josef Bacik wrote:
> We have been seeing issues in production where a cleaner script will end
> up unlinking a bunch of files that have pending iputs.  This means they
> will get their final iput's run at btrfs-cleaner time and thus are not
> throttled, which impacts the workload.
> 
> Since we are unlinking these files we can just drop the delayed iput at
> unlink time.  We are already holding a reference to the inode so this
> will not be the final iput and thus is completely safe to do at this
> point.  Doing this means we are more likely to be doing the final iput
> at unlink time, and thus will get the IO charged to the caller and get
> throttled appropriately without affecting the main workload.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

That looks a lot nicer and the explanation is sufficient.

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

> ---
> v1->v2:
> - consolidate the delayed iput run into a helper.
> 
>  fs/btrfs/inode.c | 40 ++++++++++++++++++++++++++++++++++------
>  1 file changed, 34 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 33380f5e2e8a..c311bf6d52f4 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -3326,6 +3326,28 @@ void btrfs_add_delayed_iput(struct inode *inode)
>  		wake_up_process(fs_info->cleaner_kthread);
>  }
>  
> +static void run_delayed_iput_locked(struct btrfs_fs_info *fs_info,
> +				    struct btrfs_inode *inode)
> +{
> +	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);
> +}
> +
> +static void btrfs_run_delayed_iput(struct btrfs_fs_info *fs_info,
> +				   struct btrfs_inode *inode)
> +{
> +	if (!list_empty(&inode->delayed_iput)) {
> +		spin_lock(&fs_info->delayed_iput_lock);
> +		if (!list_empty(&inode->delayed_iput))
> +			run_delayed_iput_locked(fs_info, inode);
> +		spin_unlock(&fs_info->delayed_iput_lock);
> +	}
> +}
> +
>  void btrfs_run_delayed_iputs(struct btrfs_fs_info *fs_info)
>  {
>  
> @@ -3335,12 +3357,7 @@ void btrfs_run_delayed_iputs(struct btrfs_fs_info *fs_info)
>  
>  		inode = list_first_entry(&fs_info->delayed_iputs,
>  				struct btrfs_inode, delayed_iput);
> -		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);
> +		run_delayed_iput_locked(fs_info, inode);
>  	}
>  	spin_unlock(&fs_info->delayed_iput_lock);
>  }
> @@ -4045,6 +4062,17 @@ static int __btrfs_unlink_inode(struct btrfs_trans_handle *trans,
>  		ret = 0;
>  	else if (ret)
>  		btrfs_abort_transaction(trans, ret);
> +
> +	/*
> +	 * If we have a pending delayed iput we could end up with the final iput
> +	 * being run in btrfs-cleaner context.  If we have enough of these built
> +	 * up we can end up burning a lot of time in btrfs-cleaner without any
> +	 * way to throttle the unlinks.  Since we're currently holding a ref on
> +	 * the inode we can run the delayed iput here without any issues as the
> +	 * final iput won't be done until after we drop the ref we're currently
> +	 * holding.
> +	 */
> +	btrfs_run_delayed_iput(fs_info, inode);
>  err:
>  	btrfs_free_path(path);
>  	if (ret)
>