Re: [PATCH] hwlatdetect: disable/enable c-state transitions during detection

[John Kacur <jkacur@redhat.com>]

On Mon, 6 May 2019, Clark Williams wrote:

> Recent performance tuning problems led me to realize that just running
> at fifo:99 and turning off interrupts isn't enough while looking for
> BIOS induced latencies. Power savings logic is built into most modern
> cpus and so must be disabled while looking for BIOS induced (SMI/NMI)
> latencies.
> 
> Use the /dev/cpu_dma_latency mechanism to disable c-state transitions
> while running the hardware latency detector. Open the file
> /dev/cpu_dma_latency and write a 32-bit zero to it, which will prevent
> c-state transitions while the file is open.
> 
> Signed-off-by: Clark Williams <williams@redhat.com>
> ---
>  src/hwlatdetect/hwlatdetect.py | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/src/hwlatdetect/hwlatdetect.py b/src/hwlatdetect/hwlatdetect.py
> index 2c8f9f160419..368079a158b1 100755
> --- a/src/hwlatdetect/hwlatdetect.py
> +++ b/src/hwlatdetect/hwlatdetect.py
> @@ -1,6 +1,6 @@
>  #!/usr/bin/python3
>  
> -# (C) 2018 Clark Williams <williams@redhat.com>
> +# (C) 2018,2019 Clark Williams <williams@redhat.com>
>  # (C) 2015,2016 Clark Williams <williams@redhat.com>
>  # (C) 2009 Clark Williams <williams@redhat.com>
>  #
> @@ -213,6 +213,22 @@ watch = False
>              counts = [ int(x.strip()) for x in p.stdout.readlines()]
>          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.
> +    # 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")
> +
> +    def c_states_on(self):
> +        if self.dma_latency_handle:
> +            os.close(self.dma_latency_handle)
> +        debug("c-states enabled")
> +
>      def cleanup(self):
>          raise RuntimeError("must override base method 'cleanup'!")
>  
> @@ -235,6 +251,7 @@ watch = False
>      def start(self):
>          count = 0
>          threshold = int(self.get("threshold"))
> +        self.c_states_off()
>          debug("enabling detector module (threshold: %d)" % threshold)
>          self.set("enable", 1)
>          while self.get("enable") == 0:
> @@ -258,6 +275,7 @@ watch = False
>              time.sleep(0.1)
>              debug("retrying disable of detector module(%d)" % count)
>              self.set("enable", 0)
> +        self.c_states_on()
>          debug("detector module disabled")
>  
>      def detect(self):
> -- 
> 2.21.0
> 
> 

Signed-off-by: John Kacur <jkacur@redhat.com>