[PATCH 00/17] coresight: next v4.12-rc4
Leo Yan
leo.yan at linaro.org
Thu Jun 15 08:50:54 PDT 2017
Hi Kim,
On Thu, Jun 15, 2017 at 09:22:46AM -0500, Kim Phillips wrote:
> On Thu, 15 Jun 2017 09:41:22 +0100
> Sudeep Holla <sudeep.holla at arm.com> wrote:
>
> > On 15/06/17 04:34, Kim Phillips wrote:
> > [...]
> > > How does one tell whether writes to /dev/cpu_dma_latency are effective?
> > >
> > Watch /sys/devices/system/cpu/cpu*/cpuidle/state*/{usage,time}, they
> > shouldn't tick based on the value set to cpu_dma_latency.
> >
> > E.g. a value of 0 should stop entering all the idle states.
>
> I see all states following the same pattern whether 0 or not:
>
> root at juno:~# cat show2.sh
> show()
> {
> echo $1 > /dev/cpu_dma_latency
> echo === $1:
> cat /sys/devices/system/cpu/cpu2/cpuidle/state*/{usage,time}
> echo ---
> sleep 10
> cat /sys/devices/system/cpu/cpu2/cpuidle/state*/{usage,time}
> }
>
> show 0
> show 1500
> show 0
> root at juno:~# taskset -c 2 bash ./show2.sh
> === 0:
> 1893
> 894
> 5417
> 1398675
> 65997648
> 3818880750
> ---
> 1895
> 905
> 5434
> 1401721
> 68076867
> 3826795502
> === 1500:
> 1895
> 905
> 5434
> 1401721
> 68076867
> 3826795502
> ---
> 1895
> 918
> 5447
> 1401721
> 69403510
> 3835466081
> === 0:
> 1895
> 918
> 5447
> 1401721
> 69403510
> 3835466081
> ---
> 1896
> 929
> 5463
> 1403577
> 71454839
> 3843409979
>
> googling suggests the /dev/cpu_dma_latency file should be *kept* open,
> and written a 32-bit binary number:
>
> https://access.redhat.com/articles/65410
>
> If so (I don't have time to test), this cpu debug documentation is very
> deceptive.
Looked into the code, and I must apologize I have not verified the
command when wrote the doc. As you pointed out, the closing file
operation releases the constraint for PM QoS.
Below command can work (I tested):
exec 3<> /dev/cpu_dma_latency; echo 0 >&3
Will commit one patch for this.
Thanks,
Leo Yan
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