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Diffstat (limited to 'Documentation/scheduler/schedutil.txt')
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diff --git a/Documentation/scheduler/schedutil.txt b/Documentation/scheduler/schedutil.txt deleted file mode 100644 index 78f6b91e2291..000000000000 --- a/Documentation/scheduler/schedutil.txt +++ /dev/null @@ -1,169 +0,0 @@ - - -NOTE; all this assumes a linear relation between frequency and work capacity, -we know this is flawed, but it is the best workable approximation. - - -PELT (Per Entity Load Tracking) -------------------------------- - -With PELT we track some metrics across the various scheduler entities, from -individual tasks to task-group slices to CPU runqueues. As the basis for this -we use an Exponentially Weighted Moving Average (EWMA), each period (1024us) -is decayed such that y^32 = 0.5. That is, the most recent 32ms contribute -half, while the rest of history contribute the other half. - -Specifically: - - ewma_sum(u) := u_0 + u_1*y + u_2*y^2 + ... - - ewma(u) = ewma_sum(u) / ewma_sum(1) - -Since this is essentially a progression of an infinite geometric series, the -results are composable, that is ewma(A) + ewma(B) = ewma(A+B). This property -is key, since it gives the ability to recompose the averages when tasks move -around. - -Note that blocked tasks still contribute to the aggregates (task-group slices -and CPU runqueues), which reflects their expected contribution when they -resume running. - -Using this we track 2 key metrics: 'running' and 'runnable'. 'Running' -reflects the time an entity spends on the CPU, while 'runnable' reflects the -time an entity spends on the runqueue. When there is only a single task these -two metrics are the same, but once there is contention for the CPU 'running' -will decrease to reflect the fraction of time each task spends on the CPU -while 'runnable' will increase to reflect the amount of contention. - -For more detail see: kernel/sched/pelt.c - - -Frequency- / CPU Invariance ---------------------------- - -Because consuming the CPU for 50% at 1GHz is not the same as consuming the CPU -for 50% at 2GHz, nor is running 50% on a LITTLE CPU the same as running 50% on -a big CPU, we allow architectures to scale the time delta with two ratios, one -Dynamic Voltage and Frequency Scaling (DVFS) ratio and one microarch ratio. - -For simple DVFS architectures (where software is in full control) we trivially -compute the ratio as: - - f_cur - r_dvfs := ----- - f_max - -For more dynamic systems where the hardware is in control of DVFS we use -hardware counters (Intel APERF/MPERF, ARMv8.4-AMU) to provide us this ratio. -For Intel specifically, we use: - - APERF - f_cur := ----- * P0 - MPERF - - 4C-turbo; if available and turbo enabled - f_max := { 1C-turbo; if turbo enabled - P0; otherwise - - f_cur - r_dvfs := min( 1, ----- ) - f_max - -We pick 4C turbo over 1C turbo to make it slightly more sustainable. - -r_cpu is determined as the ratio of highest performance level of the current -CPU vs the highest performance level of any other CPU in the system. - - r_tot = r_dvfs * r_cpu - -The result is that the above 'running' and 'runnable' metrics become invariant -of DVFS and CPU type. IOW. we can transfer and compare them between CPUs. - -For more detail see: - - - kernel/sched/pelt.h:update_rq_clock_pelt() - - arch/x86/kernel/smpboot.c:"APERF/MPERF frequency ratio computation." - - Documentation/scheduler/sched-capacity.rst:"1. CPU Capacity + 2. Task utilization" - - -UTIL_EST / UTIL_EST_FASTUP --------------------------- - -Because periodic tasks have their averages decayed while they sleep, even -though when running their expected utilization will be the same, they suffer a -(DVFS) ramp-up after they are running again. - -To alleviate this (a default enabled option) UTIL_EST drives an Infinite -Impulse Response (IIR) EWMA with the 'running' value on dequeue -- when it is -highest. A further default enabled option UTIL_EST_FASTUP modifies the IIR -filter to instantly increase and only decay on decrease. - -A further runqueue wide sum (of runnable tasks) is maintained of: - - util_est := \Sum_t max( t_running, t_util_est_ewma ) - -For more detail see: kernel/sched/fair.c:util_est_dequeue() - - -UCLAMP ------- - -It is possible to set effective u_min and u_max clamps on each CFS or RT task; -the runqueue keeps an max aggregate of these clamps for all running tasks. - -For more detail see: include/uapi/linux/sched/types.h - - -Schedutil / DVFS ----------------- - -Every time the scheduler load tracking is updated (task wakeup, task -migration, time progression) we call out to schedutil to update the hardware -DVFS state. - -The basis is the CPU runqueue's 'running' metric, which per the above it is -the frequency invariant utilization estimate of the CPU. From this we compute -a desired frequency like: - - max( running, util_est ); if UTIL_EST - u_cfs := { running; otherwise - - clamp( u_cfs + u_rt , u_min, u_max ); if UCLAMP_TASK - u_clamp := { u_cfs + u_rt; otherwise - - u := u_clamp + u_irq + u_dl; [approx. see source for more detail] - - f_des := min( f_max, 1.25 u * f_max ) - -XXX IO-wait; when the update is due to a task wakeup from IO-completion we -boost 'u' above. - -This frequency is then used to select a P-state/OPP or directly munged into a -CPPC style request to the hardware. - -XXX: deadline tasks (Sporadic Task Model) allows us to calculate a hard f_min -required to satisfy the workload. - -Because these callbacks are directly from the scheduler, the DVFS hardware -interaction should be 'fast' and non-blocking. Schedutil supports -rate-limiting DVFS requests for when hardware interaction is slow and -expensive, this reduces effectiveness. - -For more information see: kernel/sched/cpufreq_schedutil.c - - -NOTES ------ - - - On low-load scenarios, where DVFS is most relevant, the 'running' numbers - will closely reflect utilization. - - - In saturated scenarios task movement will cause some transient dips, - suppose we have a CPU saturated with 4 tasks, then when we migrate a task - to an idle CPU, the old CPU will have a 'running' value of 0.75 while the - new CPU will gain 0.25. This is inevitable and time progression will - correct this. XXX do we still guarantee f_max due to no idle-time? - - - Much of the above is about avoiding DVFS dips, and independent DVFS domains - having to re-learn / ramp-up when load shifts. - |