block: Track DISCARD statistics and output them in stat and diskstat

Add tracking of REQ_OP_DISCARD ios to the partition statistics and
append them to the various stat files in /sys as well as
/proc/diskstats.  These are tracked with the same four stats as reads
and writes:

Number of discard ios completed.
Number of discard ios merged
Number of discard sectors completed
Milliseconds spent on discard requests

This is done via adding a new STAT_DISCARD define to genhd.h and then
using it to index that stat field for discard requests.

tj: Refreshed on top of v4.17 and other previous updates.

Signed-off-by: Michael Callahan <michaelcallahan@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andy Newell <newella@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/ABI/testing/procfs-diskstats

@@ -5,6 +5,7 @@ Description:
 		The /proc/diskstats file displays the I/O statistics
 		of block devices. Each line contains the following 14
 		fields:
+
 		 1 - major number
 		 2 - minor mumber
 		 3 - device name
@@ -19,4 +20,13 @@ Description:
 		12 - I/Os currently in progress
 		13 - time spent doing I/Os (ms)
 		14 - weighted time spent doing I/Os (ms)
+
+		Kernel 4.18+ appends four more fields for discard
+		tracking putting the total at 18:
+
+		15 - discards completed successfully
+		16 - discards merged
+		17 - sectors discarded
+		18 - time spent discarding
+
 		For more details refer to Documentation/iostats.txt

Documentation/block/stat.txt

@@ -31,28 +31,32 @@ write ticks     milliseconds  total wait time for write requests
 in_flight       requests      number of I/Os currently in flight
 io_ticks        milliseconds  total time this block device has been active
 time_in_queue   milliseconds  total wait time for all requests
+discard I/Os    requests      number of discard I/Os processed
+discard merges  requests      number of discard I/Os merged with in-queue I/O
+discard sectors sectors       number of sectors discarded
+discard ticks   milliseconds  total wait time for discard requests
 
-read I/Os, write I/Os
-=====================
+read I/Os, write I/Os, discard I/0s
+===================================
 
 These values increment when an I/O request completes.
 
-read merges, write merges
-=========================
+read merges, write merges, discard merges
+=========================================
 
 These values increment when an I/O request is merged with an
 already-queued I/O request.
 
-read sectors, write sectors
-===========================
+read sectors, write sectors, discard_sectors
+============================================
 
-These values count the number of sectors read from or written to this
-block device.  The "sectors" in question are the standard UNIX 512-byte
-sectors, not any device- or filesystem-specific block size.  The
-counters are incremented when the I/O completes.
+These values count the number of sectors read from, written to, or
+discarded from this block device.  The "sectors" in question are the
+standard UNIX 512-byte sectors, not any device- or filesystem-specific
+block size.  The counters are incremented when the I/O completes.
 
-read ticks, write ticks
-=======================
+read ticks, write ticks, discard ticks
+======================================
 
 These values count the number of milliseconds that I/O requests have
 waited on this block device.  If there are multiple I/O requests waiting,

Documentation/iostats.txt

@@ -31,6 +31,9 @@ Here are examples of these different formats::
       3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
       3    1   hda1 35486 38030 38030 38030
 
+   4.18+ diskstats:
+      3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0
+
 On 2.4 you might execute ``grep 'hda ' /proc/partitions``. On 2.6+, you have
 a choice of ``cat /sys/block/hda/stat`` or ``grep 'hda ' /proc/diskstats``.
 
@@ -101,6 +104,18 @@ Field 11 -- weighted # of milliseconds spent doing I/Os
     last update of this field.  This can provide an easy measure of both
     I/O completion time and the backlog that may be accumulating.
 
+Field 12 -- # of discards completed
+    This is the total number of discards completed successfully.
+
+Field 13 -- # of discards merged
+    See the description of field 2
+
+Field 14 -- # of sectors discarded
+    This is the total number of sectors discarded successfully.
+
+Field 15 -- # of milliseconds spent discarding
+    This is the total number of milliseconds spent by all discards (as
+    measured from __make_request() to end_that_request_last()).
 
 To avoid introducing performance bottlenecks, no locks are held while
 modifying these counters.  This implies that minor inaccuracies may be

block/genhd.c

@@ -1333,8 +1333,11 @@ static int diskstats_show(struct seq_file *seqf, void *v)
 		part_round_stats(gp->queue, cpu, hd);
 		part_stat_unlock();
 		part_in_flight(gp->queue, hd, inflight);
-		seq_printf(seqf, "%4d %7d %s %lu %lu %lu "
-			   "%u %lu %lu %lu %u %u %u %u\n",
+		seq_printf(seqf, "%4d %7d %s "
+			   "%lu %lu %lu %u "
+			   "%lu %lu %lu %u "
+			   "%u %u %u "
+			   "%lu %lu %lu %u\n",
 			   MAJOR(part_devt(hd)), MINOR(part_devt(hd)),
 			   disk_name(gp, hd->partno, buf),
 			   part_stat_read(hd, ios[STAT_READ]),
@@ -1347,7 +1350,11 @@ static int diskstats_show(struct seq_file *seqf, void *v)
 			   jiffies_to_msecs(part_stat_read(hd, ticks[STAT_WRITE])),
 			   inflight[0],
 			   jiffies_to_msecs(part_stat_read(hd, io_ticks)),
-			   jiffies_to_msecs(part_stat_read(hd, time_in_queue))
+			   jiffies_to_msecs(part_stat_read(hd, time_in_queue)),
+			   part_stat_read(hd, ios[STAT_DISCARD]),
+			   part_stat_read(hd, merges[STAT_DISCARD]),
+			   part_stat_read(hd, sectors[STAT_DISCARD]),
+			   jiffies_to_msecs(part_stat_read(hd, ticks[STAT_DISCARD]))
 			);
 	}
 	disk_part_iter_exit(&piter);

block/partition-generic.c

@@ -130,7 +130,8 @@ ssize_t part_stat_show(struct device *dev,
 	return sprintf(buf,
 		"%8lu %8lu %8llu %8u "
 		"%8lu %8lu %8llu %8u "
-		"%8u %8u %8u"
+		"%8u %8u %8u "
+		"%8lu %8lu %8llu %8u"
 		"\n",
 		part_stat_read(p, ios[STAT_READ]),
 		part_stat_read(p, merges[STAT_READ]),
@@ -142,7 +143,11 @@ ssize_t part_stat_show(struct device *dev,
 		jiffies_to_msecs(part_stat_read(p, ticks[STAT_WRITE])),
 		inflight[0],
 		jiffies_to_msecs(part_stat_read(p, io_ticks)),
-		jiffies_to_msecs(part_stat_read(p, time_in_queue)));
+		jiffies_to_msecs(part_stat_read(p, time_in_queue)),
+		part_stat_read(p, ios[STAT_DISCARD]),
+		part_stat_read(p, merges[STAT_DISCARD]),
+		(unsigned long long)part_stat_read(p, sectors[STAT_DISCARD]),
+		jiffies_to_msecs(part_stat_read(p, ticks[STAT_DISCARD])));
 }
 
 ssize_t part_inflight_show(struct device *dev, struct device_attribute *attr,

include/linux/blk_types.h

@@ -360,6 +360,7 @@ enum req_flag_bits {
 enum stat_group {
 	STAT_READ,
 	STAT_WRITE,
+	STAT_DISCARD,
 
 	NR_STAT_GROUPS
 };
@@ -401,8 +402,15 @@ static inline bool op_is_sync(unsigned int op)
 		(op & (REQ_SYNC | REQ_FUA | REQ_PREFLUSH));
 }
 
+static inline bool op_is_discard(unsigned int op)
+{
+	return (op & REQ_OP_MASK) == REQ_OP_DISCARD;
+}
+
 static inline int op_stat_group(unsigned int op)
 {
+	if (op_is_discard(op))
+		return STAT_DISCARD;
 	return op_is_write(op);
 }
 

include/linux/genhd.h

@@ -356,7 +356,8 @@ static inline void free_part_stats(struct hd_struct *part)
 
 #define part_stat_read_accum(part, field)				\
 	(part_stat_read(part, field[STAT_READ]) +			\
-	 part_stat_read(part, field[STAT_WRITE]))
+	 part_stat_read(part, field[STAT_WRITE]) +			\
+	 part_stat_read(part, field[STAT_DISCARD]))
 
 #define part_stat_add(cpu, part, field, addnd)	do {			\
 	__part_stat_add((cpu), (part), field, addnd);			\