Re: [PATCH] btrfs: track current commit duration in commit_stats

[David Sterba <dsterba@suse.cz>]

On Thu, Apr 24, 2025 at 04:56:34PM -0700, Boris Burkov wrote:
> When debugging/detecting outlier commit stalls, having an indicator that
> we are currently in a long commit critical section can be very useful.
> Extend the commit_stats sysfs file to also include the current commit
> critical section duration.
> 
> Since this requires storing the last commit start time, use that rather
> than a separate stack variable for storing the finished commit durations
> as well.
> 
> Signed-off-by: Boris Burkov <boris@bur.io>

Reviewed-by: David Sterba <dsterba@suse.com>

> --- a/fs/btrfs/sysfs.c
> +++ b/fs/btrfs/sysfs.c
> @@ -1138,13 +1138,21 @@ static ssize_t btrfs_commit_stats_show(struct kobject *kobj,
>  				       struct kobj_attribute *a, char *buf)
>  {
>  	struct btrfs_fs_info *fs_info = to_fs_info(kobj);
> +	u64 now = ktime_get_ns();
> +	u64 start_time = fs_info->commit_stats.start_time;
> +	u64 pending = 0;
> +
> +	if (start_time)
> +		pending = now - start_time;
>  
>  	return sysfs_emit(buf,
>  		"commits %llu\n"
> +		"cur_commit_ms %llu\n"
>  		"last_commit_ms %llu\n"
>  		"max_commit_ms %llu\n"
>  		"total_commit_ms %llu\n",

This ordering looks logical, the presence of order should not be assumed
by applications, this is line oriented so a grep should always work.

>  		fs_info->commit_stats.commit_count,
> +		div_u64(pending, NSEC_PER_MSEC),
>  		div_u64(fs_info->commit_stats.last_commit_dur, NSEC_PER_MSEC),
>  		div_u64(fs_info->commit_stats.max_commit_dur, NSEC_PER_MSEC),
>  		div_u64(fs_info->commit_stats.total_commit_dur, NSEC_PER_MSEC));