Re: [PATCH v6 3/9] fs: add percpu counters for significant multigrain timestamp events

[Darrick J. Wong]

On Mon, Jul 15, 2024 at 08:48:54AM -0400, Jeff Layton wrote:
> Four percpu counters for counting various stats around mgtimes, and a
> new debugfs file for displaying them:
> 
> - number of attempted ctime updates
> - number of successful i_ctime_nsec swaps
> - number of fine-grained timestamp fetches
> - number of floor value swaps
> 
> Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  fs/inode.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 869994285e87..fff844345c35 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -21,6 +21,8 @@
>  #include <linux/list_lru.h>
>  #include <linux/iversion.h>
>  #include <linux/rw_hint.h>
> +#include <linux/seq_file.h>
> +#include <linux/debugfs.h>
>  #include <trace/events/writeback.h>
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/timestamp.h>
> @@ -80,6 +82,10 @@ EXPORT_SYMBOL(empty_aops);
>  
>  static DEFINE_PER_CPU(unsigned long, nr_inodes);
>  static DEFINE_PER_CPU(unsigned long, nr_unused);
> +static DEFINE_PER_CPU(unsigned long, mg_ctime_updates);
> +static DEFINE_PER_CPU(unsigned long, mg_fine_stamps);
> +static DEFINE_PER_CPU(unsigned long, mg_floor_swaps);
> +static DEFINE_PER_CPU(unsigned long, mg_ctime_swaps);

Should this all get switched off if CONFIG_DEBUG_FS=n?

--D

>  
>  static struct kmem_cache *inode_cachep __ro_after_init;
>  
> @@ -101,6 +107,42 @@ static inline long get_nr_inodes_unused(void)
>  	return sum < 0 ? 0 : sum;
>  }
>  
> +static long get_mg_ctime_updates(void)
> +{
> +	int i;
> +	long sum = 0;
> +	for_each_possible_cpu(i)
> +		sum += per_cpu(mg_ctime_updates, i);
> +	return sum < 0 ? 0 : sum;
> +}
> +
> +static long get_mg_fine_stamps(void)
> +{
> +	int i;
> +	long sum = 0;
> +	for_each_possible_cpu(i)
> +		sum += per_cpu(mg_fine_stamps, i);
> +	return sum < 0 ? 0 : sum;
> +}
> +
> +static long get_mg_floor_swaps(void)
> +{
> +	int i;
> +	long sum = 0;
> +	for_each_possible_cpu(i)
> +		sum += per_cpu(mg_floor_swaps, i);
> +	return sum < 0 ? 0 : sum;
> +}
> +
> +static long get_mg_ctime_swaps(void)
> +{
> +	int i;
> +	long sum = 0;
> +	for_each_possible_cpu(i)
> +		sum += per_cpu(mg_ctime_swaps, i);
> +	return sum < 0 ? 0 : sum;
> +}
> +
>  long get_nr_dirty_inodes(void)
>  {
>  	/* not actually dirty inodes, but a wild approximation */
> @@ -2655,6 +2697,7 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
>  
>  			/* Get a fine-grained time */
>  			fine = ktime_get();
> +			this_cpu_inc(mg_fine_stamps);
>  
>  			/*
>  			 * If the cmpxchg works, we take the new floor value. If
> @@ -2663,11 +2706,14 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
>  			 * as good, so keep it.
>  			 */
>  			old = floor;
> -			if (!atomic64_try_cmpxchg(&ctime_floor, &old, fine))
> +			if (atomic64_try_cmpxchg(&ctime_floor, &old, fine))
> +				this_cpu_inc(mg_floor_swaps);
> +			else
>  				fine = old;
>  			now = ktime_mono_to_real(fine);
>  		}
>  	}
> +	this_cpu_inc(mg_ctime_updates);
>  	now_ts = timestamp_truncate(ktime_to_timespec64(now), inode);
>  	cur = cns;
>  
> @@ -2682,6 +2728,7 @@ struct timespec64 inode_set_ctime_current(struct inode *inode)
>  		/* If swap occurred, then we're (mostly) done */
>  		inode->i_ctime_sec = now_ts.tv_sec;
>  		trace_ctime_ns_xchg(inode, cns, now_ts.tv_nsec, cur);
> +		this_cpu_inc(mg_ctime_swaps);
>  	} else {
>  		/*
>  		 * Was the change due to someone marking the old ctime QUERIED?
> @@ -2751,3 +2798,24 @@ umode_t mode_strip_sgid(struct mnt_idmap *idmap,
>  	return mode & ~S_ISGID;
>  }
>  EXPORT_SYMBOL(mode_strip_sgid);
> +
> +static int mgts_show(struct seq_file *s, void *p)
> +{
> +	long ctime_updates = get_mg_ctime_updates();
> +	long ctime_swaps = get_mg_ctime_swaps();
> +	long fine_stamps = get_mg_fine_stamps();
> +	long floor_swaps = get_mg_floor_swaps();
> +
> +	seq_printf(s, "%lu %lu %lu %lu\n",
> +		   ctime_updates, ctime_swaps, fine_stamps, floor_swaps);
> +	return 0;
> +}
> +
> +DEFINE_SHOW_ATTRIBUTE(mgts);
> +
> +static int __init mg_debugfs_init(void)
> +{
> +	debugfs_create_file("multigrain_timestamps", S_IFREG | S_IRUGO, NULL, NULL, &mgts_fops);
> +	return 0;
> +}
> +late_initcall(mg_debugfs_init);
> 
> -- 
> 2.45.2
> 
>