st: implement tape statistics

This patch implements tape statistics in the st module via
sysfs. Current no statistics are available for tape I/O and there
is no easy way to reuse the block layer statistics for tape
as tape is a character device and does not have perform I/O in
sector sized chunks (the size of the data written to tape
can change). For tapes we also need extra stats related to
things like tape movement (via other I/O).

There have been multiple end users requesting statistics
including AT&T (and some HP customers who have not given
permission to be named). It is impossible for them
to investigate any issues related to tape performance
in a non-invasive way.

[jejb: eliminate PRId64]
Signed-off-by: Shane Seymour <[email protected]>
Tested-by: Shane Seymour <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: James Bottomley <[email protected]>

Documentation/ABI/testing/sysfs-class-scsi_tape

@@ -0,0 +1,109 @@
+What:           /sys/class/scsi_tape/*/stats/in_flight
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Show the number of I/Os currently in-flight between the st
+		module and the SCSI mid-layer.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/io_ns
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the total amount of time spent waiting for all I/O
+                to and from the tape drive to complete. This includes all
+                reads, writes, and other SCSI commands issued to the tape
+                drive. An example of other SCSI commands would be tape
+                movement such as a rewind when a rewind tape device is
+                closed. This item is measured in nanoseconds.
+
+                To determine the amount of time spent waiting for other I/O
+                to complete subtract read_ns and write_ns from this value.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/other_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		The number of I/O requests issued to the tape drive other
+		than SCSI read/write requests.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/read_byte_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the total number of bytes requested from the tape drive.
+		This value is presented in bytes because tape drives support
+		variable length block sizes.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/read_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the total number of read requests issued to the tape
+		drive.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/read_ns
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the total amount of time in nanoseconds waiting for
+		read I/O requests to complete.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/write_byte_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the total number of bytes written to the tape drive.
+		This value is presented in bytes because tape drives support
+		variable length block sizes.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/write_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the total number of write requests issued to the tape
+		drive.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/write_ms
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the total amount of time in nanoseconds waiting for
+		write I/O requests to complete.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/resid_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <[email protected]>
+Description:
+		Shows the number of times we found that a residual >0
+		was found when the SCSI midlayer indicated that there was
+		an error. For reads this may be a case of someone issuing
+		reads greater than the block size.
+Users:

Documentation/scsi/st.txt

@@ -151,6 +151,65 @@ A link named 'tape' is made from the SCSI device directory to the class
 directory corresponding to the mode 0 auto-rewind device (e.g., st0). 
 
 
+SYSFS AND STATISTICS FOR TAPE DEVICES
+
+The st driver maintains statistics for tape drives inside the sysfs filesystem.
+The following method can be used to locate the statistics that are
+available (assuming that sysfs is mounted at /sys):
+
+1. Use opendir(3) on the directory /sys/class/scsi_tape
+2. Use readdir(3) to read the directory contents
+3. Use regcomp(3)/regexec(3) to match directory entries to the extended
+        regular expression "^st[0-9]+$"
+4. Access the statistics from the /sys/class/scsi_tape/<match>/stats
+        directory (where <match> is a directory entry from /sys/class/scsi_tape
+        that matched the extended regular expression)
+
+The reason for using this approach is that all the character devices
+pointing to the same tape drive use the same statistics. That means
+that st0 would have the same statistics as nst0.
+
+The directory contains the following statistics files:
+
+1.  in_flight - The number of I/Os currently outstanding to this device.
+2.  io_ns - The amount of time spent waiting (in nanoseconds) for all I/O
+        to complete (including read and write). This includes tape movement
+        commands such as seeking between file or set marks and implicit tape
+        movement such as when rewind on close tape devices are used.
+3.  other_cnt - The number of I/Os issued to the tape drive other than read or
+        write commands. The time taken to complete these commands uses the
+        following calculation io_ms-read_ms-write_ms.
+4.  read_byte_cnt - The number of bytes read from the tape drive.
+5.  read_cnt - The number of read requests issued to the tape drive.
+6.  read_ns - The amount of time (in nanoseconds) spent waiting for read
+        requests to complete.
+7.  write_byte_cnt - The number of bytes written to the tape drive.
+8.  write_cnt - The number of write requests issued to the tape drive.
+9.  write_ns - The amount of time (in nanoseconds) spent waiting for write
+        requests to complete.
+10. resid_cnt - The number of times during a read or write we found
+	the residual amount to be non-zero. This should mean that a program
+	is issuing a read larger thean the block size on tape. For write
+	not all data made it to tape.
+
+Note: The in_flight value is incremented when an I/O starts the I/O
+itself is not added to the statistics until it completes.
+
+The total of read_cnt, write_cnt, and other_cnt may not total to the same
+value as iodone_cnt at the device level. The tape statistics only count
+I/O issued via the st module.
+
+When read the statistics may not be temporally consistent while I/O is in
+progress. The individual values are read and written to atomically however
+when reading them back via sysfs they may be in the process of being
+updated when starting an I/O or when it is completed.
+
+The value shown in in_flight is incremented before any statstics are
+updated and decremented when an I/O completes after updating statistics.
+The value of in_flight is 0 when there are no I/Os outstanding that are
+issued by the st driver. Tape statistics do not take into account any
+I/O performed via the sg device.
+
 BSD AND SYS V SEMANTICS
 
 The user can choose between these two behaviours of the tape driver by