Re: [Patch v4 0/6] Introduce Thermal Pressure

Ionela Voinescu
2019-10-31T09:44:24Z

Hi Thara,

On Tuesday 22 Oct 2019 at 16:34:19 (-0400), Thara Gopinath wrote:
> Thermal governors can respond to an overheat event of a cpu by
> capping the cpu's maximum possible frequency. This in turn
> means that the maximum available compute capacity of the
> cpu is restricted. But today in the kernel, task scheduler is
> not notified of capping of maximum frequency of a cpu.
> In other words, scheduler is unware of maximum capacity

Nit: s/unware/unaware

> restrictions placed on a cpu due to thermal activity.
> This patch series attempts to address this issue.
> The benefits identified are better task placement among available
> cpus in event of overheating which in turn leads to better
> performance numbers.
>
> The reduction in the maximum possible capacity of a cpu due to a
> thermal event can be considered as thermal pressure. Instantaneous
> thermal pressure is hard to record and can sometime be erroneous
> as there can be mismatch between the actual capping of capacity
> and scheduler recording it. Thus solution is to have a weighted
> average per cpu value for thermal pressure over time.
> The weight reflects the amount of time the cpu has spent at a
> capped maximum frequency. Since thermal pressure is recorded as
> an average, it must be decayed periodically. Exisiting algorithm
> in the kernel scheduler pelt framework is re-used to calculate
> the weighted average. This patch series also defines a sysctl
> inerface to allow for a configurable decay period.
>
> Regarding testing, basic build, boot and sanity testing have been
> performed on db845c platform with debian file system.
> Further, dhrystone and hackbench tests have been
> run with the thermal pressure algorithm. During testing, due to
> constraints of step wise governor in dealing with big little systems,
> trip point 0 temperature was made assymetric between cpus in little
> cluster and big cluster; the idea being that
> big core will heat up and cpu cooling device will throttle the
> frequency of the big cores faster, there by limiting the maximum available
> capacity and the scheduler will spread out tasks to little cores as well.
>

Can you please share the changes you've made to sdm845.dtsi and a kernel
base on top of which to apply your patches? I would like to reproduce
your results and run more tests and it would be good if our setups were
as close as possible.

> Test Results
>
> Hackbench: 1 group , 30000 loops, 10 runs
> Result SD
> (Secs) (% of mean)
> No Thermal Pressure 14.03 2.69%
> Thermal Pressure PELT Algo. Decay : 32 ms 13.29 0.56%
> Thermal Pressure PELT Algo. Decay : 64 ms 12.57 1.56%
> Thermal Pressure PELT Algo. Decay : 128 ms 12.71 1.04%
> Thermal Pressure PELT Algo. Decay : 256 ms 12.29 1.42%
> Thermal Pressure PELT Algo. Decay : 512 ms 12.42 1.15%
>
> Dhrystone Run Time : 20 threads, 3000 MLOOPS
> Result SD
> (Secs) (% of mean)
> No Thermal Pressure 9.452 4.49%
> Thermal Pressure PELT Algo. Decay : 32 ms 8.793 5.30%
> Thermal Pressure PELT Algo. Decay : 64 ms 8.981 5.29%
> Thermal Pressure PELT Algo. Decay : 128 ms 8.647 6.62%
> Thermal Pressure PELT Algo. Decay : 256 ms 8.774 6.45%
> Thermal Pressure PELT Algo. Decay : 512 ms 8.603 5.41%
>

Do you happen to know by how much the CPUs were capped during these
experiments?

Thanks,
Ionela.

> A Brief History
>
> The first version of this patch-series was posted with resuing
> PELT algorithm to decay thermal pressure signal. The discussions
> that followed were around whether intanteneous thermal pressure
> solution is better and whether a stand-alone algortihm to accumulate
> and decay thermal pressure is more appropriate than re-using the
> PELT framework.
> Tests on Hikey960 showed the stand-alone algorithm performing slightly
> better than resuing PELT algorithm and V2 was posted with the stand
> alone algorithm. Test results were shared as part of this series.
> Discussions were around re-using PELT algorithm and running
> further tests with more granular decay period.
>
> For some time after this development was impeded due to hardware
> unavailability, some other unforseen and possibly unfortunate events.
> For this version, h/w was switched from hikey960 to db845c.
> Also Instantaneous thermal pressure was never tested as part of this
> cycle as it is clear that weighted average is a better implementation.
> The non-PELT algorithm never gave any conclusive results to prove that it
> is better than reusing PELT algorithm, in this round of testing.
> Also reusing PELT algorithm means thermal pressure tracks the
> other utilization signals in the scheduler.
>
> v3->v4:
> - "Patch 3/7:sched: Initialize per cpu thermal pressure structure"
> is dropped as it is no longer needed following changes in other
> other patches.
> - rest of the change log mentioned in specific patches.
>
> Thara Gopinath (6):
> sched/pelt.c: Add support to track thermal pressure
> sched: Add infrastructure to store and update instantaneous thermal
> pressure
> sched/fair: Enable CFS periodic tick to update thermal pressure
> sched/fair: update cpu_capcity to reflect thermal pressure
> thermal/cpu-cooling: Update thermal pressure in case of a maximum
> frequency capping
> sched: thermal: Enable tuning of decay period
>
> Documentation/admin-guide/kernel-parameters.txt | 5 ++
> drivers/thermal/cpu_cooling.c | 31 ++++++++++-
> include/linux/sched.h | 8 +++
> kernel/sched/Makefile | 2 +-
> kernel/sched/fair.c | 6 +++
> kernel/sched/pelt.c | 13 +++++
> kernel/sched/pelt.h | 7 +++
> kernel/sched/sched.h | 1 +
> kernel/sched/thermal.c | 68 +++++++++++++++++++++++++
> kernel/sched/thermal.h | 13 +++++
> 10 files changed, 151 insertions(+), 3 deletions(-)
> create mode 100644 kernel/sched/thermal.c
> create mode 100644 kernel/sched/thermal.h
>
> --
> 2.1.4
>