`Hi Vincent,On 10/19/18 6:17 PM, Vincent Guittot wrote:> The current implementation of load tracking invariance scales the> contribution with current frequency and uarch performance (only for> utilization) of the CPU. One main result of this formula is that the> figures are capped by current capacity of CPU. Another one is that the> load_avg is not invariant because not scaled with uarch.> > The util_avg of a periodic task that runs r time slots every p time slots> varies in the range :> >      U * (1-y^r)/(1-y^p) * y^i < Utilization < U * (1-y^r)/(1-y^p)> > with U is the max util_avg value = SCHED_CAPACITY_SCALE> > At a lower capacity, the range becomes:> >      U * C * (1-y^r')/(1-y^p) * y^i' < Utilization <  U * C * (1-y^r')/(1-y^p)> > with C reflecting the compute capacity ratio between current capacity and> max capacity.> > so C tries to compensate changes in (1-y^r') but it can't be accurate.> > Instead of scaling the contribution value of PELT algo, we should scale the> running time. The PELT signal aims to track the amount of computation of> tasks and/or rq so it seems more correct to scale the running time to> reflect the effective amount of computation done since the last update.> > In order to be fully invariant, we need to apply the same amount of> running time and idle time whatever the current capacity. Because running> at lower capacity implies that the task will run longer, we have to ensure> that the same amount of idle time will be apply when system becomes idle> and no idle time has been "stolen". But reaching the maximum utilization> value (SCHED_CAPACITY_SCALE) means that the task is seen as an> always-running task whatever the capacity of the CPU (even at max compute> capacity). In this case, we can discard this "stolen" idle times which> becomes meaningless.> > In order to achieve this time scaling, a new clock_pelt is created per rq.> The increase of this clock scales with current capacity when something> is running on rq and synchronizes with clock_task when rq is idle. With> this mecanism, we ensure the same running and idle time whatever the> current capacity. This also enables to simplify the pelt algorithm by> removing all references of uarch and frequency and applying the same> contribution to utilization and loads. Furthermore, the scaling is done> only once per update of clock (update_rq_clock_task()) instead of during> each update of sched_entities and cfs/rt/dl_rq of the rq like the current> implementation. This is interesting when cgroup are involved as shown in> the results below:I have a couple of questions related to the tests you ran.> On a hikey (octo ARM platform).> Performance cpufreq governor and only shallowest c-state to remove variance> generated by those power features so we only track the impact of pelt algo.So you disabled c-state 'cpu-sleep' and 'cluster-sleep'?I get 'hisi_thermal f7030700.tsensor: THERMAL ALARM: 66385 > 65000' on my hikey620. Did you change the thermal configuration? Not sure if there are any actions attached to this warning though.> each test runs 16 times> > ./perf bench sched pipe> (higher is better)> kernel	tip/sched/core     + patch>          ops/seconds        ops/seconds         diff> cgroup> root    59648(+/- 0.13%)   59785(+/- 0.24%)    +0.23%> level1  55570(+/- 0.21%)   56003(+/- 0.24%)    +0.78%> level2  52100(+/- 0.20%)   52788(+/- 0.22%)    +1.32%> > hackbench -l 1000Shouldn't this be '-l 100'?> (lower is better)> kernel	tip/sched/core     + patch>          duration(sec)      duration(sec)        diff> cgroup> root    4.472(+/- 1.86%)   4.346(+/- 2.74%)     -2.80%> level1  5.039(+/- 11.05%)  4.662(+/- 7.57%)     -7.47%> level2  5.195(+/- 10.66%)  4.877(+/- 8.90%)     -6.12%> > The responsivness of PELT is improved when CPU is not running at max> capacity with this new algorithm. I have put below some examples of> duration to reach some typical load values according to the capacity of the> CPU with current implementation and with this patch.> > Util (%)     max capacity  half capacity(mainline)  half capacity(w/ patch)> 972 (95%)    138ms         not reachable            276ms> 486 (47.5%)  30ms          138ms                     60ms> 256 (25%)    13ms           32ms                     26msCould you describe these testcases in more detail?So I assume you run one 100% task (possibly pinned to one CPU) on your hikey620 with userspace governor and for:  (1) max capacity:  echo 1200000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed  (2) half capacity:  echo 729000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeedand then you measure the time till t1 reaches 25%, 47.5% and 95% utilization?What's the initial utilization value of t1? I assume t1 starts with utilization=512 (post_init_entity_util_avg()).> On my hikey (octo ARM platform) with schedutil governor, the time to reach> max OPP when starting from a null utilization, decreases from 223ms with> current scale invariance down to 121ms with the new algorithm. For this> test, I have enable arch_scale_freq for arm64.Isn't the arch-specific arch_scale_freq_capacity() enabled by default on arm64 with cpufreq support?I would like to run the same tests so we can discuss results more easily.`