[PATCH] sched: Add schedutil overview

[Peter Zijlstra]

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 Documentation/scheduler/schedutil.txt |  168 ++++++++++++++++++++++++++++++++++
 1 file changed, 168 insertions(+)

--- /dev/null
+++ b/Documentation/scheduler/schedutil.txt
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+
+
+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 (Intel,
+ARMv8.4-AMU) we use hardware counters 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
+
+  u_clamp := clamp( u_cfs, u_min, u_max )
+
+  u := u_cfs + u_rt + 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.
+