Revert "runtime: remove GODEBUG=runtimecontentionstacks"

This reverts commit 87e930f7289136fad1310d4b63dd4127e409bac5 (CL 585639)

Reason for revert: This is part of a patch series that changed the
handling of contended lock2/unlock2 calls, reducing the maximum
throughput of contended runtime.mutex values, and causing a performance
regression on applications where that is (or became) the bottleneck.

Updates #66999
Updates #67585

Change-Id: I1e286d2a16d16e4af202cd5dc04b2d9c4ee71b32
Reviewed-on: https://go-review.googlesource.com/c/go/+/589097
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/runtime/pprof/pprof.go b/src/runtime/pprof/pprof.go
index 43ef66f0b0..be17e59875 100644
--- a/src/runtime/pprof/pprof.go
+++ b/src/runtime/pprof/pprof.go
@@ -166,6 +166,12 @@ import (
 // holds a lock for 1s while 5 other goroutines are waiting for the entire
 // second to acquire the lock, its unlock call stack will report 5s of
 // contention.
+//
+// Runtime-internal locks are always reported at the location
+// "runtime._LostContendedRuntimeLock". More detailed stack traces for
+// runtime-internal locks can be obtained by setting
+// `GODEBUG=runtimecontentionstacks=1` (see package [runtime] docs for
+// caveats).
 type Profile struct {
 	name  string
 	mu    sync.Mutex