Re: [PATCH v2] rt-tests: hwlatdetect: Gracefully handle lack of /dev/cpu_dma_latency

[John Kacur <jkacur@redhat.com>]

On Wed, 29 Sep 2021, Punit Agrawal wrote:

> Hi John,
> 
> Thanks for having a look.
> 
> John Kacur <jkacur@redhat.com> writes:
> 
> > On Tue, 28 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.
> >> 
> >> Reported-by: Suresh Hegde <suresh.c11@toshiba-tsip.com>
> >> Signed-off-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
> >> ---
> >> v1 -> v2:
> >> * Correct Suresh's email
> >> 
> >>  src/hwlatdetect/hwlatdetect.py | 7 +++++--
> >>  1 file changed, 5 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/src/hwlatdetect/hwlatdetect.py b/src/hwlatdetect/hwlatdetect.py
> >> index 12228f45f852..89537a6e371f 100755
> >> --- a/src/hwlatdetect/hwlatdetect.py
> >> +++ b/src/hwlatdetect/hwlatdetect.py
> >> @@ -216,8 +216,11 @@ class Detector(object):
> >>      # 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')
> >> +        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')
> >> +        else:
> >> +            self.dma_latency_handle = None
> >>          debug("c-states disabled")
> >>  
> >>      def c_states_on(self):
> >> -- 
> >> 2.32.0
> >> 
> >> 
> >
> > Thanks! I have a couple of nit-picky suggestions.
> >
> > If you initialize self.dma_latency_handle = None in the init method of 
> > class Detector, then you can drop the else part of your check to see if 
> > the file exists.
> 
> I can do that - but then I wonder if there is any harm in initialising
> the file handle in the init method itself too. That way
> c_states_[off|on]() can focus on doing their thing after checking the
> handle is not null.

I mean I suppose it would work, but I wouldn't consider it great style, 
I think of the init as a place for basic declarations and initializations.

Anyway, I'll have a look at whatever way you decide to go.

> 
> I'll send a patch in sometime addressing this and below comments.
> 
> >
> > Also, in the situation you describe, the debug messages should probably be 
> > hoisted up under the "if" part, and that goes for method c_states_on() 
> > too, as they are not relevant.
> >
> > You might as well fix the comments above the code too,
> > openinging -> opening
> > writeing -> writing
> > And edit that one line not to over flow 80 chars.
> >
> > Thanks!
> >
> > John
>