buildstats.bbclass: correct sampling of system stats

The last time of sampling would be updated within the SystemStats class
but not re-recorded into the datastore, leading to multiple samples being
collected in the same second in the sample function of buildstats.py.
Fix this to collect and store only one sample per second within a
certain tolerance to deal with variation in the arrival time.

This fix elimates the spikiness of sampled data, in cases where the difference
between the current and the last sample is taken. Previously, since many
samples per second were recorded, certain types of data would result in a
very small elapsed time and hence a small numerical difference. For example,
the CPU usage from /proc/stat is a running total of usage and taking the
difference between data collected 0.1 seconds apart would result in usage
appearing lower than it actually was.

(From OE-Core rev: 0e2df45ab066bb4ad2c4f8622ee9c1a8ecdea9cb)

Signed-off-by: Aryaman Gupta <aryaman.gupta@windriver.com>
Signed-off-by: Randy MacLeod <randy.macleod@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/buildstats.bbclass

@@ -285,7 +285,8 @@ python runqueue_stats () {
     if system_stats:
         # Ensure that we sample at important events.
         done = isinstance(e, bb.event.BuildCompleted)
-        system_stats.sample(e, force=done)
+        if system_stats.sample(e, force=done):
+            d.setVar('_buildstats_system_stats', system_stats)
         if done:
             system_stats.close()
             d.delVar('_buildstats_system_stats')

meta/lib/buildstats.py

@@ -51,11 +51,17 @@ class SystemStats:
         # Last time that we sampled /proc data resp. recorded disk monitoring data.
         self.last_proc = 0
         self.last_disk_monitor = 0
-        # Minimum number of seconds between recording a sample. This
-        # becames relevant when we get called very often while many
-        # short tasks get started. Sampling during quiet periods
+        # Minimum number of seconds between recording a sample. This becames relevant when we get
+        # called very often while many short tasks get started. Sampling during quiet periods
         # depends on the heartbeat event, which fires less often.
-        self.min_seconds = 1
+        # By default, the Heartbeat events occur roughly once every second but the actual time
+        # between these events deviates by a few milliseconds, in most cases. Hence
+        # pick a somewhat arbitary tolerance such that we sample a large majority
+        # of the Heartbeat events. This ignores rare events that fall outside the minimum
+        # and may lead an extra sample in a given second every so often. However, it allows for fairly
+        # consistent intervals between samples without missing many events.
+        self.tolerance = 0.01
+        self.min_seconds = 1.0 - self.tolerance
 
         self.meminfo_regex = re.compile(rb'^(MemTotal|MemFree|Buffers|Cached|SwapTotal|SwapFree):\s*(\d+)')
         self.diskstats_regex = re.compile(rb'^([hsv]d.|mtdblock\d|mmcblk\d|cciss/c\d+d\d+.*)$')
@@ -164,6 +170,12 @@ class SystemStats:
         return reduced
 
     def sample(self, event, force):
+        """
+        Collect and log proc or disk_monitor stats periodically.
+        Return True if a new sample is collected and hence the value last_proc or last_disk_monitor
+        is changed.
+        """
+        retval = False
         now = time.time()
         if (now - self.last_proc > self.min_seconds) or force:
             for filename, output, handler in self.proc_files:
@@ -187,6 +199,7 @@ class SystemStats:
                                  data +
                                  b'\n')
             self.last_proc = now
+            retval = True
 
         if isinstance(event, bb.event.MonitorDiskEvent) and \
            ((now - self.last_disk_monitor > self.min_seconds) or force):
@@ -196,3 +209,5 @@ class SystemStats:
                               for dev, sample in event.disk_usage.items()]).encode('ascii') +
                      b'\n')
             self.last_disk_monitor = now
+            retval = True
+        return retval