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

[Punit Agrawal <punitagrawal@gmail.com>]

John Kacur <jkacur@redhat.com> writes:

> On Wed, 29 Sep 2021, Punit Agrawal wrote:
>
>> 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
>> 
>> 
>
> Signed-off-by: John Kacur <jkacur@redhat.com>

Thanks for applying the patch!