Speed up cpu_frequncy() on Linux systems (giampaolo#1851)

The change is about using /proc/cpuinfo when available. It provides
cached values for frequencies and one can fill up minimum and maximum
frequency from /sys/devices/system/cpu/cpufreq/policy/* sub-system
(which is fast).

Fixes giampaolo#1851.

psutil/_pslinux.py

@@ -719,20 +719,37 @@ def cpu_stats():
         ctx_switches, interrupts, soft_interrupts, syscalls)
 
 
+def _cpu_get_cpuinfo_freq():
+    """Return current CPU frequency from cpuinfo if available.
+    """
+    ret = []
+    with open_binary('%s/cpuinfo' % get_procfs_path()) as f:
+        for line in f:
+            if line.lower().startswith(b'cpu mhz'):
+                ret.append(float(line.split(b':', 1)[1]))
+    return ret
+
+
 if os.path.exists("/sys/devices/system/cpu/cpufreq/policy0") or \
         os.path.exists("/sys/devices/system/cpu/cpu0/cpufreq"):
     def cpu_freq():
         """Return frequency metrics for all CPUs.
         Contrarily to other OSes, Linux updates these values in
         real-time.
         """
+        cpuinfo_freqs = _cpu_get_cpuinfo_freq()
         paths = sorted(
             glob.glob("/sys/devices/system/cpu/cpufreq/policy[0-9]*") or
             glob.glob("/sys/devices/system/cpu/cpu[0-9]*/cpufreq"))
         ret = []
         pjoin = os.path.join
-        for path in paths:
-            curr = cat(pjoin(path, "scaling_cur_freq"), fallback=None)
+        for i, path in enumerate(paths):
+            if len(paths) == len(cpuinfo_freqs):
+                # take cached value from cpuinfo if available, see:
+                # https://github.com/giampaolo/psutil/issues/1851
+                curr = cpuinfo_freqs[i]
+            else:
+                curr = cat(pjoin(path, "scaling_cur_freq"), fallback=None)
             if curr is None:
                 # Likely an old RedHat, see:
                 # https://github.com/giampaolo/psutil/issues/1071
@@ -746,24 +763,12 @@ def cpu_freq():
             ret.append(_common.scpufreq(curr, min_, max_))
         return ret
 
-elif os.path.exists("/proc/cpuinfo"):
+else:
     def cpu_freq():
         """Alternate implementation using /proc/cpuinfo.
         min and max frequencies are not available and are set to None.
         """
-        ret = []
-        with open_binary('%s/cpuinfo' % get_procfs_path()) as f:
-            for line in f:
-                if line.lower().startswith(b'cpu mhz'):
-                    key, value = line.split(b':', 1)
-                    ret.append(_common.scpufreq(float(value), 0., 0.))
-        return ret
-
-else:
-    def cpu_freq():
-        """Dummy implementation when none of the above files are present.
-        """
-        return []
+        return [_common.scpufreq(value, 0., 0.) for value in _cpu_get_cpuinfo_freq()]
 
 
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