runtime: use cgroup CPU limit to set GOMAXPROCS

This CL adds two related features enabled by default via compatibility
GODEBUGs containermaxprocs and updatemaxprocs.

On Linux, containermaxprocs makes the Go runtime consider cgroup CPU
bandwidth limits (quota/period) when setting GOMAXPROCS. If the cgroup
limit is lower than the number of logical CPUs available, then the
cgroup limit takes precedence.

On all OSes, updatemaxprocs makes the Go runtime periodically
recalculate the default GOMAXPROCS value and update GOMAXPROCS if it has
changed. If GOMAXPROCS is set manually, this update does not occur. This
is intended primarily to detect changes to cgroup limits, but it applies
on all OSes because the CPU affinity mask can change as well.

The runtime only considers the limit in the leaf cgroup (the one that
actually contains the process), caching the CPU limit file
descriptor(s), which are periodically reread for updates. This is a
small departure from the original proposed design. It will not consider
limits of parent cgroups (which may be lower than the leaf), and it will
not detection cgroup migration after process start.

We can consider changing this in the future, but the simpler approach is
less invasive; less risk to packages that have some awareness of runtime
internals. e.g., if the runtime periodically opens new files during
execution, file descriptor leak detection is difficult to implement in a
stable way.

For #73193.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Change-Id: I6a6a636c631c1ae577fb8254960377ba91c5dc98
Reviewed-on: https://go-review.googlesource.com/c/go/+/670497
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 79 insertions, 0 deletions

diff --git a/src/runtime/float.go b/src/runtime/float.go
index 9f281c4045..d8573c103b 100644
--- a/src/runtime/float.go
+++ b/src/runtime/float.go
@@ -6,6 +6,12 @@ package runtime
 
 import "unsafe"
 
+const (
+	float64Mask  = 0x7FF
+	float64Shift = 64 - 11 - 1
+	float64Bias  = 1023
+)
+
 var inf = float64frombits(0x7FF0000000000000)
 
 // isNaN reports whether f is an IEEE 754 “not-a-number” value.
@@ -52,3 +58,76 @@ func float64bits(f float64) uint64 {
 func float64frombits(b uint64) float64 {
 	return *(*float64)(unsafe.Pointer(&b))
 }
+
+// floor returns the greatest integer value less than or equal to x.
+//
+// Special cases are:
+//
+//	floor(±0) = ±0
+//	floor(±Inf) = ±Inf
+//	floor(NaN) = NaN
+//
+// N.B. Portable floor copied from math. math also has optimized arch-specific
+// implementations.
+func floor(x float64) float64 {
+	if x == 0 || isNaN(x) || isInf(x) {
+		return x
+	}
+	if x < 0 {
+		d, fract := modf(-x)
+		if fract != 0.0 {
+			d = d + 1
+		}
+		return -d
+	}
+	d, _ := modf(x)
+	return d
+}
+
+// ceil returns the least integer value greater than or equal to x.
+//
+// Special cases are:
+//
+//	Ceil(±0) = ±0
+//	Ceil(±Inf) = ±Inf
+//	Ceil(NaN) = NaN
+//
+// N.B. Portable ceil copied from math. math also has optimized arch-specific
+// implementations.
+func ceil(x float64) float64 {
+	return -floor(-x)
+}
+
+// modf returns integer and fractional floating-point numbers
+// that sum to f. Both values have the same sign as f.
+//
+// Special cases are:
+//
+//	Modf(±Inf) = ±Inf, NaN
+//	Modf(NaN) = NaN, NaN
+//
+// N.B. Portable modf copied from math. math also has optimized arch-specific
+// implementations.
+func modf(f float64) (int float64, frac float64) {
+	if f < 1 {
+		switch {
+		case f < 0:
+			int, frac = modf(-f)
+			return -int, -frac
+		case f == 0:
+			return f, f // Return -0, -0 when f == -0
+		}
+		return 0, f
+	}
+
+	x := float64bits(f)
+	e := uint(x>>float64Shift)&float64Mask - float64Bias
+
+	// Keep the top 12+e bits, the integer part; clear the rest.
+	if e < 64-12 {
+		x &^= 1<<(64-12-e) - 1
+	}
+	int = float64frombits(x)
+	frac = f - int
+	return
+}