runtime: add the disablethp GODEBUG setting

Go 1.21.1 and Go 1.22 have ceased working around an issue with Linux
kernel defaults for transparent huge pages that can result in excessive
memory overheads. (https://bugzilla.kernel.org/show_bug.cgi?id=93111)

Many Linux distributions disable huge pages altogether these days, so
this problem isn't quite as far-reaching as it used to be. Also, the
problem only affects Go programs with very particular memory usage
patterns.

That being said, because the runtime used to actively deal with this
problem (but with some unpredictable behavior), it's preventing users
that don't have a lot of control over their execution environment from
upgrading to Go beyond Go 1.20.

This change adds a GODEBUG to smooth over the transition. The GODEBUG
setting disables transparent huge pages for all heap memory on Linux,
which is much more predictable than restoring the old behavior.

Fixes golang#64332.

Change-Id: I73b1894337f0f0b1a5a17b90da1221e118e0b145
Reviewed-on: https://go-review.googlesource.com/c/go/+/547475
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

doc/godebug.md

@@ -165,6 +165,19 @@ when the connection supports neither TLS 1.3 nor Extended Master Secret
 (implemented in Go 1.21). It can be reenabled with the [`tlsunsafeekm`
 setting](/pkg/crypto/tls/#ConnectionState.ExportKeyingMaterial).
 
+Go 1.22 changed how the runtime interacts with transparent huge pages on Linux.
+In particular, a common default Linux kernel configuration can result in
+significant memory overheads, and Go 1.22 no longer works around this default.
+To work around this issue without adjusting kernel settings, transparent huge
+pages can be disabled for Go memory with the
+[`disablethp` setting](/pkg/runtime#hdr-Environment_Variable).
+This behavior was backported to Go 1.21.1, but the setting is only available
+starting with Go 1.21.6.
+This setting may be removed in a future release, and users impacted by this issue
+should adjust their Linux configuration according to the recommendations in the
+[GC guide](/doc/gc-guide#Linux_transparent_huge_pages), or switch to a Linux
+distribution that disables transparent huge pages altogether.
+
 ### Go 1.21
 
 Go 1.21 made it a run-time error to call `panic` with a nil interface value,

src/runtime/extern.go

@@ -55,6 +55,13 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	cgocheck mode can be enabled using GOEXPERIMENT (which
 	requires a rebuild), see https://pkg.go.dev/internal/goexperiment for details.
 
+	disablethp: setting disablethp=1 on Linux disables transparent huge pages for the heap.
+	It has no effect on other platforms. disablethp is meant for compatibility with versions
+	of Go before 1.21, which stopped working around a Linux kernel default that can result
+	in significant memory overuse. See https://go.dev/issue/64332. This setting will be
+	removed in a future release, so operators should tweak their Linux configuration to suit
+	their needs before then. See https://go.dev/doc/gc-guide#Linux_transparent_huge_pages.
+
 	dontfreezetheworld: by default, the start of a fatal panic or throw
 	"freezes the world", preempting all threads to stop all running
 	goroutines, which makes it possible to traceback all goroutines, and

src/runtime/mem_linux.go

@@ -170,4 +170,12 @@ func sysMapOS(v unsafe.Pointer, n uintptr) {
 		print("runtime: mmap(", v, ", ", n, ") returned ", p, ", ", err, "\n")
 		throw("runtime: cannot map pages in arena address space")
 	}
+
+	// Disable huge pages if the GODEBUG for it is set.
+	//
+	// Note that there are a few sysHugePage calls that can override this, but
+	// they're all for GC metadata.
+	if debug.disablethp != 0 {
+		sysNoHugePageOS(v, n)
+	}
 }

src/runtime/runtime1.go

@@ -309,6 +309,7 @@ type dbgVar struct {
 var debug struct {
 	cgocheck            int32
 	clobberfree         int32
+	disablethp          int32
 	dontfreezetheworld  int32
 	efence              int32
 	gccheckmark         int32
@@ -344,6 +345,7 @@ var dbgvars = []*dbgVar{
 	{name: "allocfreetrace", value: &debug.allocfreetrace},
 	{name: "clobberfree", value: &debug.clobberfree},
 	{name: "cgocheck", value: &debug.cgocheck},
+	{name: "disablethp", value: &debug.disablethp},
 	{name: "dontfreezetheworld", value: &debug.dontfreezetheworld},
 	{name: "efence", value: &debug.efence},
 	{name: "gccheckmark", value: &debug.gccheckmark},