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

[John Kacur <jkacur@redhat.com>]

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>