runtime: make piController much more defensive about overflow

If something goes horribly wrong with the assumptions surrounding a
piController, its internal error state might accumulate in an unbounded
manner. In practice this means unexpected Inf and NaN values.

Avoid this by identifying cases where the error overflows and resetting
controller state.

In the scavenger, this case is much more likely. All that has to happen
is the proportional relationship between sleep time and estimated CPU
usage has to break down. Unfortunately because we're just measuring
monotonic time for all this, there are lots of ways it could happen,
especially in an oversubscribed system. In these cases, just fall back
on a conservative pace for scavenging and try to wait out the issue.

In the pacer I'm pretty sure this is impossible. Because we wire the
output of the controller to the input, the response is very directly
correlated, so it's impossible for the controller's core assumption to
break down.

While we're in the pacer, add more detail about why that controller is
even there, as well as its purpose.

Finally, let's be proactive about other sources of overflow, namely
overflow from a very large input value. This change adds a check after
the first few operations to detect overflow issues from the input,
specifically the multiplication.

No tests for the pacer because I was unable to actually break the
pacer's controller under a fuzzer, and no tests for the scavenger because
it is not really in a testable state.

However:
* This change includes a fuzz test for the piController.
* I broke out the scavenger code locally and fuzz tested it, confirming
  that the patch eliminates the original failure mode.
* I tested that on a local heap-spike test, the scavenger continues
  operating as expected under normal conditions.

Fixes #51061.

Change-Id: I02a01d2dbf0eb9d2a8a8e7274d4165c2b6a3415a
Reviewed-on: https://go-review.googlesource.com/c/go/+/383954
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/export_test.go

@@ -1332,3 +1332,21 @@ func Releasem() {
 }
 
 var Timediv = timediv
+
+type PIController struct {
+	piController
+}
+
+func NewPIController(kp, ti, tt, min, max float64) *PIController {
+	return &PIController{piController{
+		kp:  kp,
+		ti:  ti,
+		tt:  tt,
+		min: min,
+		max: max,
+	}}
+}
+
+func (c *PIController) Next(input, setpoint, period float64) (float64, bool) {
+	return c.piController.next(input, setpoint, period)
+}