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

@@ -672,41 +672,43 @@ def cpu_stats():
         ctx_switches, interrupts, soft_interrupts, syscalls)
 
 
-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.
-        """
-        def get_path(num):
-            for p in ("/sys/devices/system/cpu/cpufreq/policy%s" % num,
-                      "/sys/devices/system/cpu/cpu%s/cpufreq" % num):
-                if os.path.exists(p):
-                    return p
+HAS_CPUFREQ = (os.path.exists("/sys/devices/system/cpu/cpufreq/policy0") or
+               os.path.exists("/sys/devices/system/cpu/cpu0/cpufreq"))
 
-        ret = []
-        for n in range(cpu_count_logical()):
-            path = get_path(n)
-            if not path:
-                continue
 
-            pjoin = os.path.join
-            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
-                curr = cat(pjoin(path, "cpuinfo_cur_freq"), fallback=None)
-                if curr is None:
-                    raise NotImplementedError(
-                        "can't find current frequency file")
-            curr = int(curr) / 1000
-            max_ = int(cat(pjoin(path, "scaling_max_freq"))) / 1000
-            min_ = int(cat(pjoin(path, "scaling_min_freq"))) / 1000
-            ret.append(_common.scpufreq(curr, min_, max_))
-        return ret
+def _get_path(num):
+    """Return frequency metrics for all CPUs.
+    Contrarily to other OSes, Linux updates these values in
+    real-time.
+    """
+    for p in ("/sys/devices/system/cpu/cpufreq/policy%s" % num,
+              "/sys/devices/system/cpu/cpu%s/cpufreq" % num):
+        if os.path.exists(p):
+            return p
+
+
+def _get_cpu_scaling_value(num, key):
+    """Return frequncy value based on key for a NUMth CPU. """
+    if not HAS_CPUFREQ:
+        return 0.
+
+    path = _get_path(num)
+    if not path:
+        return 0.
+
+    pjoin = os.path.join
+    value = cat(pjoin(path, key), fallback=None)
+    if value is None and key == "scaling_cur_freq":
+        # Likely an old RedHat, see:
+        # https://github.com/giampaolo/psutil/issues/1071
+        value = cat(pjoin(path, key), fallback=None)
+        if value is None:
+            raise NotImplementedError(
+                "can't find current frequency file")
+    return int(value) / 1000
+
 
-elif os.path.exists("/proc/cpuinfo"):
+if os.path.exists("/proc/cpuinfo"):
     def cpu_freq():
         """Alternate implementation using /proc/cpuinfo.
         min and max frequencies are not available and are set to None.
@@ -716,7 +718,20 @@ def cpu_freq():
             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.))
+                    n = len(ret)
+                    max_ = _get_cpu_scaling_value(n, "scaling_max_freq")
+                    min_ = _get_cpu_scaling_value(n, "scaling_min_freq")
+                    ret.append(_common.scpufreq(float(value), min_, max_))
+        return ret
+
+elif HAS_CPUFREQ:
+    def cpu_freq():
+        ret = []
+        for n in range(cpu_count_logical()):
+            curr = _get_cpu_scaling_value(n, "scaling_cur_freq")
+            max_ = _get_cpu_scaling_value(n, "scaling_max_freq")
+            min_ = _get_cpu_scaling_value(n, "scaling_min_freq")
+            ret.append(_common.scpufreq(curr, min_, max_))
         return ret
 
 else: