runtime: improve work stealing randomness

For certain values of GOMAXPROCS, the current code is less random than
it looks. For example with GOMAXPROCS=12, there are 4 coprimes: 1 5 7 11.
That's bad, as 12 and 4 are not relatively prime. So if pos == 2, then we
always pick 7 as the inc. We want to pick pos and inc independently
at random.

Change-Id: I5c7e4f01f9223cbc2db12a685dc0bced2cf39abf
Reviewed-on: https://go-review.googlesource.com/c/go/+/369976
Run-TryBot: Keith Randall <khr@golang.org>
Trust: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/proc.go

@@ -6138,7 +6138,7 @@ func (ord *randomOrder) start(i uint32) randomEnum {
 	return randomEnum{
 		count: ord.count,
 		pos:   i % ord.count,
-		inc:   ord.coprimes[i%uint32(len(ord.coprimes))],
+		inc:   ord.coprimes[i/ord.count%uint32(len(ord.coprimes))],
 	}
 }
 

src/runtime/proc_runtime_test.go

@@ -30,4 +30,21 @@ func RunStealOrderTest() {
 			}
 		}
 	}
+	// Make sure that different arguments to ord.start don't generate the
+	// same pos+inc twice.
+	for procs := 2; procs <= 64; procs++ {
+		ord.reset(uint32(procs))
+		checked := make([]bool, procs*procs)
+		// We want at least procs*len(ord.coprimes) different pos+inc values
+		// before we start repeating.
+		for i := 0; i < procs*len(ord.coprimes); i++ {
+			enum := ord.start(uint32(i))
+			j := enum.pos*uint32(procs) + enum.inc
+			if checked[j] {
+				println("procs:", procs, "pos:", enum.pos, "inc:", enum.inc)
+				panic("duplicate pos+inc during enumeration")
+			}
+			checked[j] = true
+		}
+	}
 }