[PATCH v2] rt-tests: hwlatdetect: Add timestamp delta

[Costa Shulyupin <costa.shul@redhat.com>]

Add delta field that calculates the time interval between consecutive
samples. For the first sample, the previous timestamp is initialized to
'nan' (Not a Number), so the first delta is also 'nan'.

This helps identify periodic issues during hardware latency testing.

v2:
Address John Kacur's review comments:
- Use float('nan') instead of string 'nan' for prev initialization
- Compute float(ts) once and reuse for delta calculation and prev update

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
---
 src/hwlatdetect/hwlatdetect.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/hwlatdetect/hwlatdetect.py b/src/hwlatdetect/hwlatdetect.py
index 42b9f301718a..18ca930487f0 100755
--- a/src/hwlatdetect/hwlatdetect.py
+++ b/src/hwlatdetect/hwlatdetect.py
@@ -253,7 +253,8 @@     def detect(self):
 
     class Sample:
         'private class for tracer sample data'
-        __slots__ = 'cpu', 'timestamp', 'inner', 'outer', 'count'
+        __slots__ = 'cpu', 'timestamp', 'delta', 'inner', 'outer', 'count'
+        prev = float('nan')
 
         def __init__(self, line):
             fields = line.split()
@@ -276,14 +277,16 @@         def __init__(self, line):
             i, o = fields[6].split('/')
             ts = fields[7][3:]
             self.timestamp = str(ts)
+            ts_float = float(ts)
+            self.delta = ts_float - self.__class__.prev
+            self.__class__.prev = ts_float
             self.inner = int(i)
             self.outer = int(o)
             self.count = int(kv["count"]) if "count" in kv else None
 
         def __str__(self):
-            if self.count is not None:
-                return f"ts: {self.timestamp}, inner:{self.inner}, outer:{self.outer}, cpu:{self.cpu}, count:{self.count}"
-            return f"ts: {self.timestamp}, inner:{self.inner}, outer:{self.outer}, cpu:{self.cpu}"
+            s = f"ts: {self.timestamp}, delta:{self.delta:.6f}, inner:{self.inner}, outer:{self.outer}, cpu:{self.cpu}"
+            return s if self.count is None else s + f", count:{self.count}"
 
         def display(self):
             """ convert object to string and print """
-- 
2.53.0