Re: [PATCH 07/13] btrfs: kick off async delayed ref flushing if we are over time budget

From: Nikolay Borisov <nborisov@suse.com>
To: Josef Bacik <josef@toxicpanda.com>,
	linux-btrfs@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 07/13] btrfs: kick off async delayed ref flushing if we are over time budget
Date: Thu, 9 Apr 2020 16:11:26 +0300	[thread overview]
Message-ID: <7b611adf-3bba-a2d3-d6eb-592bb15ce97e@suse.com> (raw)
In-Reply-To: <20200313212330.149024-8-josef@toxicpanda.com>

On 13.03.20 г. 23:23 ч., Josef Bacik wrote:
> For very large file systems we cannot rely on the space reservation
> system to provide enough pressure to flush delayed refs in a timely
> manner.  We have the infrastructure in place to keep track of how much
> theoretical time it'll take to run our outstanding delayed refs, but
> unfortunately I ripped all of that out when I added the delayed refs
> rsv.  This code originally was added to address the problem of too many
> delayed refs building up and thus causing transaction commits to take
> several minutes to finish.
> 
> Fix this by adding back the ability to flush delayed refs based on the
> time budget for them.  We want to limit to around 1 seconds worth of
> delayed refs to be pending at any given time.  In order to keep up with
> demand we will start the async flusher once we are at the 500ms mark,
> and the async flusher will attempt to keep us in this ballpark.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/ctree.h       |  4 ++++
>  fs/btrfs/disk-io.c     |  3 +++
>  fs/btrfs/extent-tree.c | 44 ++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/transaction.c |  8 ++++++++
>  4 files changed, 59 insertions(+)
> 

<snip>

>  
> +static void btrfs_async_run_delayed_refs(struct work_struct *work)
> +{
> +	struct btrfs_fs_info *fs_info;
> +	struct btrfs_trans_handle *trans;
> +
> +	fs_info = container_of(work, struct btrfs_fs_info,
> +			       async_delayed_ref_work);
> +
> +	while (!btrfs_fs_closing(fs_info)) {
> +		unsigned long count;
> +		int ret;
> +
> +		trans = btrfs_attach_transaction(fs_info->extent_root);
> +		if (IS_ERR(trans))
> +			break;
> +
> +		smp_rmb();
> +		if (trans->transaction->delayed_refs.flushing) {
> +			btrfs_end_transaction(trans);
> +			break;
> +		}
> +
> +		/* No longer over our threshold, lets bail. */
> +		if (!btrfs_should_throttle_delayed_refs(trans, true)) {
> +			btrfs_end_transaction(trans);
> +			break;
> +		}
> +
> +		count = atomic_read(&trans->transaction->delayed_refs.num_entries);

Don't you want to actually read num_heads_ready rather than num_entries,
i.e isn't this introducing the same issues as the one fixed by:

[PATCH 2/5] btrfs: delayed refs pre-flushing should only run the heads
we have

> +		count >>= 2;
> +
> +		ret = btrfs_run_delayed_refs(trans, count);
> +		btrfs_end_transaction(trans);
> +		if (ret < 0)
> +			break;
> +	}
> +}

<snip>