linux-kernel - Re: [PATCH 1/6] writeback: collect stats of all wb of bdi in bdi_debug_stats

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Date: Thu, 21 Mar 2024 19:06:20 +0100
From: Jan Kara <jack@...e.cz>
To: Kemeng Shi <shikemeng@...weicloud.com>
Cc: akpm@...ux-foundation.org, tj@...nel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	willy@...radead.org, bfoster@...hat.com, jack@...e.cz,
	dsterba@...e.com, mjguzik@...il.com, dhowells@...hat.com,
	peterz@...radead.org
Subject: Re: [PATCH 1/6] writeback: collect stats of all wb of bdi in
 bdi_debug_stats_show

On Wed 20-03-24 19:02:17, Kemeng Shi wrote:
> /sys/kernel/debug/bdi/xxx/stats is supposed to show writeback information
> of whole bdi, but only writeback information of bdi in root cgroup is
> collected. So writeback information in non-root cgroup are missing now.
> To be more specific, considering following case:
> 
> /* create writeback cgroup */
> cd /sys/fs/cgroup
> echo "+memory +io" > cgroup.subtree_control
> mkdir group1
> cd group1
> echo $$ > cgroup.procs
> /* do writeback in cgroup */
> fio -name test -filename=/dev/vdb ...
> /* get writeback info of bdi */
> cat /sys/kernel/debug/bdi/xxx/stats
> The cat result unexpectedly implies that there is no writeback on target
> bdi.
> 
> Fix this by collecting stats of all wb in bdi instead of only wb in
> root cgroup.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>

Looks mostly good, one comment below:

> ---
>  mm/backing-dev.c | 93 ++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 70 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 5f2be8c8df11..788702b6c5dd 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -39,6 +39,19 @@ struct workqueue_struct *bdi_wq;
>  #include <linux/debugfs.h>
>  #include <linux/seq_file.h>
>  
> +struct wb_stats {
> +	unsigned long nr_dirty;
> +	unsigned long nr_io;
> +	unsigned long nr_more_io;
> +	unsigned long nr_dirty_time;
> +	unsigned long nr_writeback;
> +	unsigned long nr_reclaimable;
> +	unsigned long nr_dirtied;
> +	unsigned long nr_written;
> +	unsigned long dirty_thresh;
> +	unsigned long wb_thresh;
> +};
> +
>  static struct dentry *bdi_debug_root;
>  
>  static void bdi_debug_init(void)
> @@ -46,31 +59,65 @@ static void bdi_debug_init(void)
>  	bdi_debug_root = debugfs_create_dir("bdi", NULL);
>  }
>  
> -static int bdi_debug_stats_show(struct seq_file *m, void *v)
> +static void collect_wb_stats(struct wb_stats *stats,
> +			     struct bdi_writeback *wb)
>  {
> -	struct backing_dev_info *bdi = m->private;
> -	struct bdi_writeback *wb = &bdi->wb;
> -	unsigned long background_thresh;
> -	unsigned long dirty_thresh;
> -	unsigned long wb_thresh;
> -	unsigned long nr_dirty, nr_io, nr_more_io, nr_dirty_time;
>  	struct inode *inode;
>  
> -	nr_dirty = nr_io = nr_more_io = nr_dirty_time = 0;
>  	spin_lock(&wb->list_lock);
>  	list_for_each_entry(inode, &wb->b_dirty, i_io_list)
> -		nr_dirty++;
> +		stats->nr_dirty++;
>  	list_for_each_entry(inode, &wb->b_io, i_io_list)
> -		nr_io++;
> +		stats->nr_io++;
>  	list_for_each_entry(inode, &wb->b_more_io, i_io_list)
> -		nr_more_io++;
> +		stats->nr_more_io++;
>  	list_for_each_entry(inode, &wb->b_dirty_time, i_io_list)
>  		if (inode->i_state & I_DIRTY_TIME)
> -			nr_dirty_time++;
> +			stats->nr_dirty_time++;
>  	spin_unlock(&wb->list_lock);
>  
> +	stats->nr_writeback += wb_stat(wb, WB_WRITEBACK);
> +	stats->nr_reclaimable += wb_stat(wb, WB_RECLAIMABLE);
> +	stats->nr_dirtied += wb_stat(wb, WB_DIRTIED);
> +	stats->nr_written += wb_stat(wb, WB_WRITTEN);
> +	stats->wb_thresh += wb_calc_thresh(wb, stats->dirty_thresh);
> +}
> +
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +static void bdi_collect_stats(struct backing_dev_info *bdi,
> +			      struct wb_stats *stats)
> +{
> +	struct bdi_writeback *wb;
> +
> +	/* protect wb from release */
> +	mutex_lock(&bdi->cgwb_release_mutex);
> +	list_for_each_entry(wb, &bdi->wb_list, bdi_node)
> +		collect_wb_stats(stats, wb);
> +	mutex_unlock(&bdi->cgwb_release_mutex);
> +}

So AFAICT this function can race against
  bdi_unregister() -> wb_shutdown(&bdi->wb)

because that doesn't take the cgwb_release_mutex. So we either need the RCU
protection as Brian suggested or cgwb_lock or something. But given
collect_wb_stats() can take a significant amount of time (traversing all
the lists etc.) I think we'll need something more clever.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR