[PATCH v3] rt-tests: hwlatdetect: Gracefully handle lack of /dev/cpu_dma_latency

Punit Agrawal
2021-09-29T00:37:19Z

From: Punit Agrawal <punit1.agrawal@toshiba.co.jp>

On systems where cpu idle is disabled to reduce latencies,
"/dev/cpu_dma_latency" does not exist and leads to the following
exception report from python when using hwlatdetect.py -

FileNotFoundError: [Errno 2] No such file or directory: '/dev/cpu_dma_latency

Update hwlatdetect to check whether the file exists before attemping
to write values to it. Also, make the related debug output conditional
to c-states support being enabled in the kernel.

While we are touching this part of the code, also address a couple of
minor issues in the code such as -
* correct typos
* reflow comment to fit 80-char column limit

Reported-by: Suresh Hegde <suresh.c11@toshiba-tsip.com>
Signed-off-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
---
v2 -> v3:
* Address issues in comment
* Initialise self.dma_latency_handle in init()
* Make debug() statements conditional to /dev/cpu_dma_latency
---
src/hwlatdetect/hwlatdetect.py | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/src/hwlatdetect/hwlatdetect.py b/src/hwlatdetect/hwlatdetect.py
index 12228f45f852..56fd2af8641d 100755
--- a/src/hwlatdetect/hwlatdetect.py
+++ b/src/hwlatdetect/hwlatdetect.py
@@ -200,6 +200,7 @@ class Detector(object):
if os.path.exists('/usr/sbin/rdmsr'):
self.have_msr = True
self.initsmi = self.getsmicounts()
+ self.dma_latency_handle = None

def getsmicounts(self):
counts = []
@@ -210,20 +211,23 @@ class Detector(object):
return counts

# methods for preventing/enabling c-state transitions
- # openinging /dev/cpu_dma_latency and writeing a 32-bit zero to that file will prevent
- # c-state transitions while the file descriptor is open.
+ #
+ # opening /dev/cpu_dma_latency and writing a 32-bit zero to that file will
+ # prevent c-state transitions while the file descriptor is open.
+ #
# use c_states_off() to disable c-state transitions
# use c_states_on() to close the file descriptor and re-enable c-states
#
def c_states_off(self):
- self.dma_latency_handle = os.open("/dev/cpu_dma_latency", os.O_WRONLY)
- os.write(self.dma_latency_handle, b'\x00\x00\x00\x00')
- debug("c-states disabled")
+ if os.path.exists("/dev/cpu_dma_latency"):
+ self.dma_latency_handle = os.open("/dev/cpu_dma_latency", os.O_WRONLY)
+ os.write(self.dma_latency_handle, b'\x00\x00\x00\x00')
+ debug("c-states disabled")

def c_states_on(self):
if self.dma_latency_handle:
os.close(self.dma_latency_handle)
- debug("c-states enabled")
+ debug("c-states enabled")

def cleanup(self):
raise RuntimeError("must override base method 'cleanup'!")
--
2.32.0