Messages in this thread | | | Subject | Re: [PATCH v5 2/2] sched/fair: update scale invariance of PELT | From | Dietmar Eggemann <> | Date | Fri, 2 Nov 2018 16:36:35 +0100 |
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On 10/26/18 6:11 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.
Thinking about this new approach on a big.LITTLE platform:
CPU Capacities big: 1024 LITTLE: 512, performance CPUfreq governor
A 50% (runtime/period) task on a big CPU will become an always running task on the little CPU. The utilization signal of the task and the cfs_rq of the little CPU converges to 1024.
With contrib scaling the utilization signal of the 50% task converges to 512 on the little CPU, even it is always running on it, and so does the one of the cfs_rq.
Two 25% tasks on a big CPU will become two 50% tasks on a little CPU. The utilization signal of the tasks converges to 512 and the one of the cfs_rq of the little CPU converges to 1024.
With contrib scaling the utilization signal of the 25% tasks converges to 256 on the little CPU, even they run each 50% on it, and the one of the cfs_rq converges to 512.
So what do we consider system-wide invariance? I thought that e.g. a 25% task should have a utilization value of 256 no matter on which CPU it is running?
In both cases, the little CPU is not going idle whereas the big CPU does.
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