Hi, On Tue, 20 Nov 2018 at 11:55, Vincent Guittot <vincent.guit...@linaro.org> 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 applied 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 mechanism, 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: > > On a hikey (octo Arm64 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. > > each test runs 16 times > > ./perf bench sched pipe > (higher is better) > kernel tip/sched/core + patch > ops/seconds ops/seconds diff > cgroup > root 59652(+/- 0.18%) 59876(+/- 0.24%) +0.38% > level1 55608(+/- 0.27%) 55923(+/- 0.24%) +0.57% > level2 52115(+/- 0.29%) 52564(+/- 0.22%) +0.86% > > hackbench -l 1000 > (lower is better) > kernel tip/sched/core + patch > duration(sec) duration(sec) diff > cgroup > root 4.453(+/- 2.37%) 4.383(+/- 2.88%) -1.57% > level1 4.859(+/- 8.50%) 4.830(+/- 7.07%) -0.60% > level2 5.063(+/- 9.83%) 4.928(+/- 9.66%) -2.66% > > Then, the responsiveness 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. These values has been > computed based on the geometric series and the half period value: > > 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 26ms > > On my hikey (octo Arm64 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. > > Signed-off-by: Vincent Guittot <vincent.guit...@linaro.org>
Is there anything else that I should do for these patches ? Regards, Vincent