runtime: profile contended lock calls

Add runtime-internal locks to the mutex contention profile.

Store up to one call stack responsible for lock contention on the M,
until it's safe to contribute its value to the mprof table. Try to use
that limited local storage space for a relatively large source of
contention, and attribute any contention in stacks we're not able to
store to a sentinel _LostContendedLock function.

Avoid ballooning lock contention while manipulating the mprof table by
attributing to that sentinel function any lock contention experienced
while reporting lock contention.

Guard collecting real call stacks with GODEBUG=profileruntimelocks=1,
since the available data has mixed semantics; we can easily capture an
M's own wait time, but we'd prefer for the profile entry of each
critical section to describe how long it made the other Ms wait. It's
too late in the Go 1.22 cycle to make the required changes to
futex-based locks. When not enabled, attribute the time to the sentinel
function instead.

Fixes #57071

Change-Id: I3eee0ccbfc20f333b56f20d8725dfd7f3a526b41
Reviewed-on: https://go-review.googlesource.com/c/go/+/528657
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@justin.tv>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 22 insertions, 20 deletions

diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index 489dcdd79c..877d94eef2 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -307,26 +307,27 @@ type dbgVar struct {
 // existing int var for that value, which may
 // already have an initial value.
 var debug struct {
-	cgocheck           int32
-	clobberfree        int32
-	dontfreezetheworld int32
-	efence             int32
-	gccheckmark        int32
-	gcpacertrace       int32
-	gcshrinkstackoff   int32
-	gcstoptheworld     int32
-	gctrace            int32
-	invalidptr         int32
-	madvdontneed       int32 // for Linux; issue 28466
-	scavtrace          int32
-	scheddetail        int32
-	schedtrace         int32
-	tracebackancestors int32
-	asyncpreemptoff    int32
-	harddecommit       int32
-	adaptivestackstart int32
-	tracefpunwindoff   int32
-	traceadvanceperiod int32
+	cgocheck            int32
+	clobberfree         int32
+	dontfreezetheworld  int32
+	efence              int32
+	gccheckmark         int32
+	gcpacertrace        int32
+	gcshrinkstackoff    int32
+	gcstoptheworld      int32
+	gctrace             int32
+	invalidptr          int32
+	madvdontneed        int32 // for Linux; issue 28466
+	profileruntimelocks atomic.Int32
+	scavtrace           int32
+	scheddetail         int32
+	schedtrace          int32
+	tracebackancestors  int32
+	asyncpreemptoff     int32
+	harddecommit        int32
+	adaptivestackstart  int32
+	tracefpunwindoff    int32
+	traceadvanceperiod  int32
 
 	// debug.malloc is used as a combined debug check
 	// in the malloc function and should be set
@@ -352,6 +353,7 @@ var dbgvars = []*dbgVar{
 	{name: "gctrace", value: &debug.gctrace},
 	{name: "invalidptr", value: &debug.invalidptr},
 	{name: "madvdontneed", value: &debug.madvdontneed},
+	{name: "profileruntimelocks", atomic: &debug.profileruntimelocks},
 	{name: "sbrk", value: &debug.sbrk},
 	{name: "scavtrace", value: &debug.scavtrace},
 	{name: "scheddetail", value: &debug.scheddetail},