[PATCH 6/7] dm: track the maximum number of bios in a cloned request

[Mike Snitzer <snitzer@redhat.com>]

If /sys/modules/dm_mod/parameters/track_peak_rq_based_ios is set to 1 it
enables the tracking of the maximum number of bios in a cloned request.

The maximum number of bios in a cloned request is then exposed through
the following read-only parameter:
/sys/modules/dm_mod/parameters/peak_rq_based_ios

This information can be useful when deciding on a value for dm_mod's
'reserved_rq_based_ios' parameter.  Otherwise, 'track_peak_rq_based_ios'
should be kept disabled (the default).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 186f77d..de83930 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -239,6 +239,12 @@ static unsigned reserved_rq_based_ios;
 static unsigned reserved_rq_based_ios_latch;
 
 /*
+ * Optionally track the maximum numbers of IOs in a cloned request.
+ */
+static unsigned track_peak_rq_based_ios;
+static unsigned peak_rq_based_ios;
+
+/*
  * This mutex protects reserved_bio_based_ios_latch and reserved_rq_based_ios_latch.
  */
 static DEFINE_MUTEX(dm_mempools_lock);
@@ -1719,6 +1725,16 @@ static struct request *clone_rq(struct request *rq, struct mapped_device *md,
 		return NULL;
 	}
 
+	if (unlikely(ACCESS_ONCE(track_peak_rq_based_ios))) {
+		struct bio *bio;
+		unsigned num_bios = 0;
+
+		__rq_for_each_bio(bio, clone)
+			num_bios++;
+		if (num_bios > ACCESS_ONCE(peak_rq_based_ios))
+			ACCESS_ONCE(peak_rq_based_ios) = num_bios;
+	}
+
 	return clone;
 }
 
@@ -3034,6 +3050,12 @@ MODULE_PARM_DESC(reserved_bio_based_ios, "Reserved IOs in bio-based mempools");
 module_param(reserved_rq_based_ios, uint, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(reserved_rq_based_ios, "Reserved IOs in request-based mempools");
 
+module_param(track_peak_rq_based_ios, uint, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(peak_rq_based_ios, "Enable tracking the maximum IOs in a cloned request");
+
+module_param(peak_rq_based_ios, uint, S_IRUGO);
+MODULE_PARM_DESC(peak_rq_based_ios, "Tracks the maximum IOs in a cloned request");
+
 MODULE_DESCRIPTION(DM_NAME " driver");
 MODULE_AUTHOR("Joe Thornber <dm-devel@redhat.com>");
 MODULE_LICENSE("GPL");
-- 
1.8.1.4