sched: track the runnable average on a per-task entity basis

Instead of tracking averaging the load parented by a cfs_rq, we can track
entity load directly.  With the load for a given cfs_rq then being the sum of
its children.

To do this we represent the historical contribution to runnable average within
each trailing 1024us of execution as the coefficients of a geometric series.

We can express this for a given task t as:
  runnable_sum(t) = \Sum u_i * y^i, runnable_avg_period(t) = \Sum 1024 * y^i
  load(t) = weight_t * runnable_sum(t) / runnable_avg_period(t)

Where: u_i is the usage in the last i`th 1024us period (approximately 1ms) ~ms
and y is chosen such that y^k = 1/2.  We currently choose k to be 32 which
roughly translates to about a sched period.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: maintain per-rq runnable averages

Since runqueues do not have a corresponding sched_entity we instead embed a
sched_avg structure directly.

Signed-off-by: Ben Segall <bsegall@google.com>
Reviewed-by: Paul Turner <pjt@google.com>
modified for Mako kernel from kernel.org

Signed-off-by: faux123 <reioux@gmail.com>
sched: aggregate load contributed by task entities on parenting cfs_rq

For a given task t, we can compute its contribution to load as:
  task_load(t) = runnable_avg(t) * weight(t)

On a parenting cfs_rq we can then aggregate
  runnable_load(cfs_rq) = \Sum task_load(t), for all runnable children t

Maintain this bottom up, with task entities adding their contributed load to
the parenting cfs_rq sum.  When a task entity's load changes we add the same
delta to the maintained sum.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: maintain the load contribution of blocked entities

We are currently maintaining:
  runnable_load(cfs_rq) = \Sum task_load(t)

For all running children t of cfs_rq.  While this can be naturally updated for
tasks in a runnable state (as they are scheduled); this does not account for
the load contributed by blocked task entities.

This can be solved by introducing a separate accounting for blocked load:
  blocked_load(cfs_rq) = \Sum runnable(b) * weight(b)

Obviously we do not want to iterate over all blocked entities to account for
their decay, we instead observe that:
  runnable_load(t) = \Sum p_i*y^i

and that to account for an additional idle period we only need to compute:
  y*runnable_load(t).

This means that we can compute all blocked entities at once by evaluating:
  blocked_load(cfs_rq)` = y * blocked_load(cfs_rq)

Finally we maintain a decay counter so that when a sleeping entity re-awakens
we can determine how much of its load should be removed from the blocked sum.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: add an rq migration call-back to sched_class

Since we are now doing bottom up load accumulation we need explicit
notification when a task has been re-parented so that the old hierarchy can be
updated.

Adds: migrate_task_rq(struct task_struct *p, int next_cpu)

(The alternative is to do this out of __set_task_cpu, but it was suggested that
this would be a cleaner encapsulation.)

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: account for blocked load waking back up

When a running entity blocks we migrate its tracked load to
cfs_rq->blocked_runnable_avg.  In the sleep case this occurs while holding
rq->lock and so is a natural transition.  Wake-ups however, are potentially
asynchronous in the presence of migration and so special care must be taken.

We use an atomic counter to track such migrated load, taking care to match this
with the previously introduced decay counters so that we don't migrate too much
load.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: aggregate total task_group load

Maintain a global running sum of the average load seen on each cfs_rq belonging
to each task group so that it may be used in calculating an appropriate
shares:weight distribution.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: compute load contribution by a group entity

Unlike task entities who have a fixed weight, group entities instead own a
fraction of their parenting task_group's shares as their contributed weight.

Compute this fraction so that we can correctly account hierarchies and shared
entity nodes.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: normalize tg load contributions against runnable time

Entities of equal weight should receive equitable distribution of cpu time.
This is challenging in the case of a task_group's shares as execution may be
occurring on multiple cpus simultaneously.

To handle this we divide up the shares into weights proportionate with the load
on each cfs_rq.  This does not however, account for the fact that the sum of
the parts may be less than one cpu and so we need to normalize:
  load(tg) = min(runnable_avg(tg), 1) * tg->shares
Where runnable_avg is the aggregate time in which the task_group had runnable
children.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>.
sched: maintain runnable averages across throttled periods

With bandwidth control tracked entities may cease execution according to user
specified bandwidth limits.  Charging this time as either throttled or blocked
however, is incorrect and would falsely skew in either direction.

What we actually want is for any throttled periods to be "invisible" to
load-tracking as they are removed from the system for that interval and
contribute normally otherwise.

Do this by moderating the progression of time to omit any periods in which the
entity belonged to a throttled hierarchy.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: replace update_shares weight distribution with per-entity computation

Now that the machinery in place is in place to compute contributed load in a
bottom up fashion; replace the shares distribution code within update_shares()
accordingly.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: refactor update_shares_cpu() -> update_blocked_avgs()

Now that running entities maintain their own load-averages the work we must do
in update_shares() is largely restricted to the periodic decay of blocked
entities.  This allows us to be a little less pessimistic regarding our
occupancy on rq->lock and the associated rq->clock updates required.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: update_cfs_shares at period edge

Now that our measurement intervals are small (~1ms) we can amortize the posting
of update_shares() to be about each period overflow.  This is a large cost
saving for frequently switching tasks.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: make __update_entity_runnable_avg() fast

__update_entity_runnable_avg forms the core of maintaining an entity's runnable
load average.  In this function we charge the accumulated run-time since last
update and handle appropriate decay.  In some cases, e.g. a waking task, this
time interval may be much larger than our period unit.

Fortunately we can exploit some properties of our series to perform decay for a
blocked update in constant time and account the contribution for a running
update in essentially-constant* time.

[*]: For any running entity they should be performing updates at the tick which
gives us a soft limit of 1 jiffy between updates, and we can compute up to a
32 jiffy update in a single pass.

C program to generate the above:

#include <math.h>
#include <stdio.h>

#define N 32
#define WMULT_SHIFT 32

const long WMULT_CONST = ((1UL << N) - 1);
double y;

long runnable_avg_yN_inv[N];
void calc_mult_inv() {
	int i;
	double yn = 0;

	printf("inverses\n");
	for (i = 0; i < N; i++) {
		yn = (double)WMULT_CONST * pow(y, i);
		runnable_avg_yN_inv[i] = yn;
		printf("%2d: 0x%8lx\n", i, runnable_avg_yN_inv[i]);
	}
	printf("\n");
}

long mult_inv(long c, int n) {
	return (c * runnable_avg_yN_inv[n]) >> WMULT_SHIFT;
}

void calc_yn_sum(int n)
{
	int i;
	double sum = 0, sum_fl = 0, diff = 0;

	/*
	 * We take the floored sum to ensure the sum of partial sums is never
	 * larger than the actual sum.
	 */
	printf("sum y^n\n");
	printf("   %8s  %8s %8s\n", "exact", "floor", "error");
	for (i = 1; i <= n; i++) {
		sum = (y * sum + y * 1024);
		sum_fl = floor(y * sum_fl+ y * 1024);
		printf("%2d: %8.0f  %8.0f %8.0f\n", i, sum, sum_fl,
			sum_fl - sum);
	}
	printf("\n");
}

void calc_conv(long n) {
	long old_n;
	int i = -1;

	printf("convergence (LOAD_AVG_MAX, LOAD_AVG_MAX_N)\n");
	do {
		old_n = n;
		n = mult_inv(n, 1) + 1024;
		i++;
	} while (n != old_n);
	printf("%d> %ld\n", i - 1, n);
	printf("\n");
}

void main() {
	y = pow(0.5, 1/(double)N);
	calc_mult_inv();
	calc_conv(1024);
	calc_yn_sum(N);
}

[ Compile with -lm ]
Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>
sched: implement usage tracking

With the frame-work for runnable tracking now fully in place.  Per-entity usage
tracking is a simple and low-overhead addition.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>