[Devel] [PATCH RHEL10 COMMIT] drivers/md/dm-stats: add rd_histogram option

[Konstantin Khorenko]

The commit is pushed to "branch-rh10-6.12.0-211.39.1.16.x.vz10-ovz" and will appear at git at bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-211.39.1.16.1.vz10
------>
commit ecd0dbc260e03cc02f2027925b0a11b2d763dd0c
Author: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
Date:   Mon Jul 27 19:32:50 2026 +0300

    drivers/md/dm-stats: add rd_histogram option
    
    Allow a region to collect a latency histogram that only accounts
    reads.
    It can be combined with the existing "histogram:" argument within
    one region.
    
    https://virtuozzo.atlassian.net/browse/VSTOR-103846
    Feature: dm-stats: latency and histogram enhancements
    Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
    Reviewed-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
    Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 .../admin-guide/device-mapper/statistics.rst        | 10 ++++++++--
 drivers/md/dm-stats.c                               | 21 +++++++++++++++++++--
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/device-mapper/statistics.rst b/Documentation/admin-guide/device-mapper/statistics.rst
index d5b1b1013c087..28e2f9c49ddcb 100644
--- a/Documentation/admin-guide/device-mapper/statistics.rst
+++ b/Documentation/admin-guide/device-mapper/statistics.rst
@@ -95,6 +95,12 @@ Messages
 		that took 10-20 ms to complete, c is the number of requests
 		that took 20-30 ms to complete and d is the number of
 		requests that took more than 30 ms to complete.
+	  rd_histogram:n1,n2,n3,n4,...
+		collect a latency histogram like "histogram", but only
+		account read requests.
+	  Multiple histogram arguments may be specified for one region;
+	  each histogram is reported as an additional counter set in the
+	  @stats_print output, in the order the histograms were specified.
 
 	<program_id>
 	  An optional parameter.  A name that uniquely identifies
@@ -139,8 +145,8 @@ Messages
 	        precise_timestamps hist_total_latency histogram:n1,n2,n3,...
 
 	The strings "precise_timestamps", "hist_total_latency" and
-	"histogram" are printed only if they were specified when creating
-	the region.
+	"histogram" (also "rd_histogram") are printed only if they were
+	specified when creating the region.
 
     @stats_print <region_id> [<starting_line> <number_of_lines>]
 	Print counters for each step-sized area of a region.
diff --git a/drivers/md/dm-stats.c b/drivers/md/dm-stats.c
index e55b821fb9a34..46d4468294197 100644
--- a/drivers/md/dm-stats.c
+++ b/drivers/md/dm-stats.c
@@ -52,7 +52,7 @@ enum {
 #define DM_STAT_CLASSES_DATA	((1 << DM_STAT_CLASS_READ) | \
 				 (1 << DM_STAT_CLASS_WRITE))
 
-#define DM_STAT_MAX_HISTOGRAMS		1
+#define DM_STAT_MAX_HISTOGRAMS		2
 
 struct dm_stat_histogram {
 	unsigned int class_mask;	/* classes this histogram accounts */
@@ -541,6 +541,16 @@ static int dm_stats_delete(struct dm_stats *stats, int id)
 	return 0;
 }
 
+static const char *dm_stat_histogram_name(unsigned int class_mask)
+{
+	switch (class_mask) {
+	case 1 << DM_STAT_CLASS_READ:
+		return "rd_histogram";
+	default:
+		return "histogram";
+	}
+}
+
 static int dm_stats_list(struct dm_stats *stats, const char *program,
 			 char *result, unsigned int maxlen)
 {
@@ -572,7 +582,8 @@ static int dm_stats_list(struct dm_stats *stats, const char *program,
 				struct dm_stat_histogram *h = &s->histograms[hn];
 				unsigned int i;
 
-				DMEMIT(" histogram:");
+				DMEMIT(" %s:",
+				       dm_stat_histogram_name(h->class_mask));
 				for (i = 0; i < h->n_entries; i++) {
 					if (i)
 						DMEMIT(",");
@@ -1188,6 +1199,12 @@ static int message_stats_create(struct mapped_device *md,
 						  DM_STAT_CLASSES_DATA, a + 10);
 				if (r)
 					goto ret;
+			} else if (!strncasecmp(a, "rd_histogram:", 13)) {
+				r = add_histogram(histograms, &n_histograms,
+						  1 << DM_STAT_CLASS_READ,
+						  a + 13);
+				if (r)
+					goto ret;
 			} else
 				goto ret_einval;
 		}