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