rename bad funcs instead of removing their samples

Removing the samples for functions that have known inlining issues skews
the PGO profile and might cause previously cold functions to be promoted
to hot functions.

It's probably not a big issue, but this is an easy improvement to make,
so why not.

Kudos to Michael Pratt for pointing this out pointing this out in
#3 (comment)

noinline.go

@@ -6,30 +6,34 @@ import (
 	"github.com/google/pprof/profile"
 )
 
-const grpcProcessDataFunc = "google.golang.org/grpc/internal/transport.(*loopyWriter).processData"
+const (
+	grpcProcessDataFunc = "google.golang.org/grpc/internal/transport.(*loopyWriter).processData"
+	doNotInlinePrefix   = "DO NOT INLINE: "
+)
 
-// ApplyNoInlineHack removes problematic samples from the profile to avoid bad
+// ApplyNoInlineHack renames problematic functions in the profile to avoid bad
 // inlining decisions that can have a large memory impact.
 // See https://github.com/golang/go/issues/65532 for details.
 //
 // TODO: Delete this once it's fixed upstream.
 func ApplyNoInlineHack(prof *profile.Profile) error {
-	if err := removeLeafSamples(prof, []string{grpcProcessDataFunc}); err != nil {
+	if err := renameNoInlineFuncs(prof, []string{grpcProcessDataFunc}); err != nil {
 		return fmt.Errorf("noinline hack: %w", err)
 	}
 	return nil
 }
 
-func removeLeafSamples(prof *profile.Profile, funcs []string) error {
-	newSamples := make([]*profile.Sample, 0, len(prof.Sample))
+func renameNoInlineFuncs(prof *profile.Profile, noInlineFuncs []string) error {
 	for _, s := range prof.Sample {
 		leaf, ok := leafLine(s)
-		if ok && lineContainsAny(leaf, funcs) {
-			continue
+		if ok && lineContainsAny(leaf, noInlineFuncs) {
+			// There might be multiple samples that point to the same function.
+			// But once we rename the function for the function for the first
+			// time, it stops matching the noInlineFuncs list. So we don't end
+			// up adding the prefix multiple times.
+			leaf.Function.Name = doNotInlinePrefix + leaf.Function.Name
 		}
-		newSamples = append(newSamples, s)
 	}
-	prof.Sample = newSamples
 	return nil
 }
 

noinline_test.go

@@ -14,6 +14,7 @@ func TestApplyNoInlineHack(t *testing.T) {
 	require.Equal(t, 17, leafSamples(prof, grpcProcessDataFunc))
 	require.NoError(t, ApplyNoInlineHack(prof))
 	require.Equal(t, 0, leafSamples(prof, grpcProcessDataFunc))
+	require.Equal(t, 17, leafSamples(prof, doNotInlinePrefix+grpcProcessDataFunc))
 }
 
 func loadTestProfile(t *testing.T, name string) *profile.Profile {