Re: [RFC PATCH] thermal: add generic cpu hotplug cooling device

[Zoran Markovic]

Hi Eduardo,
The merge window for 3.14 is now open and I'm wondering if you had a
chance to look at these numbers?
Thanks,
Zoran

On 30 December 2013 12:48, Zoran Markovic <zoran.markovic@xxxxxxxxxx> wrote:
> Eduardo,
>
>>> What is the workload you're running besides the proprietary heater code?
> I re-did experiments from Linaro's site pointed by Amit while
> profiling _cpu_down() and _cpu_up() times:
>>> [1] https://wiki.linaro.org/WorkingGroups/PowerManagement/Archives/Hotplug
>
> I am attaching a spreadsheet with some results and graphs:
>
> Sheet 1 (thermal_ramp) has three plots. Topmost is an unbound thermal
> ramp that levels off at ~48C. Middle plot is a thermal ramp with cpu
> hotplug kicking in as a cooling device at 38C. Bottom plot is a
> thermal ramp with cpu hotplug kicking in at 38C and cpufreq kicking in
> at 40C. One interesting thing to note is that the middle plot slowly
> drifts towards 40C even though cooling is set to 38C. I attribute this
> to the logic of step-wise governor combined with polling mode: if
> temperature is dropping above trip point, cooling is reduced. Adding
> another cooling device at 40C as a back-stop seems to keep temperature
> in check. In all cases running code was ARM's max_power test that
> maximizes CPU usage, as evidenced by results of 'top':
>   PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
>    33 root      20   0     0    0    0 R 100.0  0.0  45:46.43 thread1
>    32 root      20   0     0    0    0 R  91.4  0.0  44:48.14 thread0
>  1344 root      20   0     0    0    0 R   8.6  0.0   0:03.64 kworker/u4:1
>  1380 root      20   0  2476  996  712 R   0.3  0.1   0:00.07 top
>
> Sheet 2 (idle) has two plots. Top one represents latency of
> _cpu_down() while gradually adding instances of cyclictest process,
> from 0 to 10; 20 samples were captured in each case. Bottom one
> represents latency of _cpu_up() in the same test. Other than running
> cyclictest, the system was mostly idle.
>
> Sheet 3 (max_power) repeated the same test as in sheet 2, but it was
> running ARM's max_power test in the background.
>
> A quick look at the latency graphs shows that loading the system
> causes a stochastic - but not deterministic - component added to
> latencies. Minimum latency times appear unchanged.
>
>> - Homogeneous dual core Cortex-A9 environment.
>> - They go up to 48C when fully loaded. Can you explain where is your
>> sensor location? Gradient to hotspot, etc? 48C at A9s or board temperature?
> Thermal sensor is located at L2 cache, with gradient to sensor likely
> smaller than sensor inaccuracy.
>
>> - This code looks promising on embedded dual core system. However, it
>> does not necessarily mean it works fine on, say server side. How about a
>> system with 8/16/32 cores? How about a more heterogeneous workload? Not
>> to talk about heterogeneous cores. I think in more complicated scenarios
>> the data you provided above might even change. The difference between
>> your minimum and maximum shutdown/startup times are quite considerable,
>> so I am assuming your variance is not negligible, imaging if we scale
>> this up, what happens?
> Agreed that this is difficult to characterize across all platform
> types. Maybe other list members could comment the behaviour on their
> platforms? Passing in a cpu mask defines CPUs that contribute to
> cooling of a single zone, so there is some flexibility in defining
> cooling strategy. Hopefully this is good enough for a start...
>
>>
>> - The other point is that this type of cooling device must be taken in
>> very sensible way. Shutting down circuitry may not be the best strategy
>> for thermal. In fact, if you think about it, given you have a workload
>> well balanced between, say, two cores, as same of your environment,
>> turning one off it means you need to deal the very same load in only one
>> CPU. In other words, turning of circuitry means, from thermal standpoint
>> that you are increasing you heat/area ratio. Sometimes, you actually
>> want to increase this ratio in order to properly cool down your system.
> In this particular test case since both CPUs are fully loaded,
> temperature is reduced at the expense of parallelism (i.e. execution
> time), so overall heat/area is still reduced. If particular areas are
> heat-sensitive, then it makes sense to define a separate thermal zone
> (and sensor) for each of them. Just a thought.
>
> Looking forward to further discussion.
>
> Regards,
> Zoran
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