introduce the log_issue_time option

introduce the log_issue_time option

When write_lat_log option is set, fio outputs the 'clat' completion
latency log file. This clat log can be used to analyze IO latency. This
file can also be considered as an IO trace file as each IO entry can
fully describe an IO when the --log_offset and --log_prio options are
also used.

However, using the clat log file as an IO trace is inaccuate due to two
reasons. Firstly, the time field of each entry uses millisecond units,
which is too coarse for fast IOs (e.g. when using SSDs). Secondly, the
time field value is recorded not at command completion, but at log
sample recording. The time field value is slightly different from the IO
completion time. It can be used only as an approximated completion time.

To analyze IO issue time and IO completion time accurately using the
clat log, introduce the new option 'log_issue_time'. When this option is
set, add another field to the log file entries and put the IO issue time
in nanosecond to the field. The IO completion time can be calculated by
adding the completion latency to the IO issue time.

The IO issue time field is added to 'slat' submit latency log file also.
This helps to calculate IO start time by subtracting the submission
latency from the IO issue time.

The log_issue_time option can be used for IO trace when the
write_lat_log option and the log_offset options are set together. When
the log_issue_time option is set but the write_lat_log option or the
log_offset option is not set, fio errors out. When the log_issue_time
option and the write_lat_log option are set together with other
write_X_log options, the IO issue time field is added to all log files.
As for the other log files than clat and slat log, the IO issue time
does not have meaning then '0' is set to the field. When log_avg_msec
option is set, average of the log values of the specified duration is
logged. The IO issue time does not have meaning in this case either and
'0' is set to the field.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20240829085826.999859-6-shinichiro.kawasaki@wdc.com
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>