runtime: document maxStack and m.createstack in more details

Change-Id: If93b6cfa5a598a5f4101c879a0cd88a194e4a6aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/518116
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Andy Pan <[email protected]>

Author: panjf2000

src/runtime/mprof.go

@@ -39,7 +39,10 @@ const (
 	// size of bucket hash table
 	buckHashSize = 179999
 
-	// max depth of stack to record in bucket
+	// maxStack is the max depth of stack to record in bucket.
+	// Note that it's only used internally as a guard against
+	// wildly out-of-bounds slicing of the PCs that come after
+	// a bucket struct, and it could increase in the future.
 	maxStack = 32
 )
 

src/runtime/runtime2.go

@@ -583,7 +583,7 @@ type m struct {
 	alllink       *m // on allm
 	schedlink     muintptr
 	lockedg       guintptr
-	createstack   [32]uintptr // stack that created this thread.
+	createstack   [32]uintptr // stack that created this thread, it's used for StackRecord.Stack0, so it must align with it.
 	lockedExt     uint32      // tracking for external LockOSThread
 	lockedInt     uint32      // tracking for internal lockOSThread
 	nextwaitm     muintptr    // next m waiting for lock