runtime: remove VDSO fallback test and benchmarks

These tests assume that it is OK to switch between time implementations,
but the clock_gettime call uses CLOCK_MONOTONIC and the fallback call,
gettimeofday, uses CLOCK_REALTIME. Disabling the clock_gettime call means
that calls to nanotime will start returning very different values.
This breaks the new timer code, which assumes that nanotime will return
a consistently increasing value.

This test is not very useful in any case as it doesn't check the results.

Removing this file also removes BenchmarkTimeNow, which is a duplicate
of BenchmarkNow in the time package.

Updates #27707
Fixes #32109

Change-Id: I6a884af07f75822d724193c5eed94742f524f07d
Reviewed-on: https://go-review.googlesource.com/c/go/+/174679Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/vdso_linux_test.go

-// Copyright 2017 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build linux
-// +build 386 amd64 arm arm64 ppc64 ppc64le
-
-package runtime_test
-
-import (
-	"testing"
-	"time"
-	_ "unsafe"
-)
-
-// These tests are a little risky because they overwrite the vdsoClockgettimeSym value.
-// It's normally initialized at startup and remains unchanged after that.
-
-//go:linkname vdsoClockgettimeSym runtime.vdsoClockgettimeSym
-var vdsoClockgettimeSym uintptr
-
-func TestClockVDSOAndFallbackPaths(t *testing.T) {
-	// Check that we can call walltime() and nanotime() with and without their (1st) fast-paths.
-	// This just checks that fast and fallback paths can be called, rather than testing their
-	// results.
-	//
-	// Call them indirectly via time.Now(), so we don't need auxiliary .s files to allow us to
-	// use go:linkname to refer to the functions directly.
-
-	save := vdsoClockgettimeSym
-	if save == 0 {
-		t.Log("vdsoClockgettime symbol not found; fallback path will be used by default")
-	}
-
-	// Call with fast-path enabled (if vDSO symbol found at startup)
-	time.Now()
-
-	// Call with fast-path disabled
-	vdsoClockgettimeSym = 0
-	time.Now()
-	vdsoClockgettimeSym = save
-}
-
-func BenchmarkClockVDSOAndFallbackPaths(b *testing.B) {
-	run := func(b *testing.B) {
-		for i := 0; i < b.N; i++ {
-			// Call via time.Now() - see comment in test above.
-			time.Now()
-		}
-	}
-
-	save := vdsoClockgettimeSym
-	b.Run("vDSO", run)
-	vdsoClockgettimeSym = 0
-	b.Run("Fallback", run)
-	vdsoClockgettimeSym = save
-}
-
-func BenchmarkTimeNow(b *testing.B) {
-	for i := 0; i < b.N; i++ {
-		time.Now()
-	}
-}