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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
> +# (C) 2018,2019 Clark Williams <williams@xxxxxxxxxx>
> # (C) 2015,2016 Clark Williams <williams@xxxxxxxxxx>
> # (C) 2009 Clark Williams <williams@xxxxxxxxxx>
> #
> @@ -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@xxxxxxxxxx>
