runtime: deduplicate pMask resize code

Change-Id: I04a9a69904710a488c685cb9eee9c3313ed8e97b
Reviewed-on: https://go-review.googlesource.com/c/go/+/701896
Reviewed-by: Michael Pratt <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mknyszek

src/runtime/proc.go

@@ -5873,8 +5873,6 @@ func procresize(nprocs int32) *p {
 	}
 	sched.procresizetime = now
 
-	maskWords := (nprocs + 31) / 32
-
 	// Grow allp if necessary.
 	if nprocs > int32(len(allp)) {
 		// Synchronize with retake, which could be running
@@ -5890,19 +5888,8 @@ func procresize(nprocs int32) *p {
 			allp = nallp
 		}
 
-		if maskWords <= int32(cap(idlepMask)) {
-			idlepMask = idlepMask[:maskWords]
-			timerpMask = timerpMask[:maskWords]
-		} else {
-			nidlepMask := make([]uint32, maskWords)
-			// No need to copy beyond len, old Ps are irrelevant.
-			copy(nidlepMask, idlepMask)
-			idlepMask = nidlepMask
-
-			ntimerpMask := make([]uint32, maskWords)
-			copy(ntimerpMask, timerpMask)
-			timerpMask = ntimerpMask
-		}
+		idlepMask = idlepMask.resize(nprocs)
+		timerpMask = timerpMask.resize(nprocs)
 		unlock(&allpLock)
 	}
 
@@ -5965,8 +5952,8 @@ func procresize(nprocs int32) *p {
 	if int32(len(allp)) != nprocs {
 		lock(&allpLock)
 		allp = allp[:nprocs]
-		idlepMask = idlepMask[:maskWords]
-		timerpMask = timerpMask[:maskWords]
+		idlepMask = idlepMask.resize(nprocs)
+		timerpMask = timerpMask.resize(nprocs)
 		unlock(&allpLock)
 	}
 
@@ -6905,6 +6892,22 @@ func (p pMask) clear(id int32) {
 	atomic.And(&p[word], ^mask)
 }
 
+// resize resizes the pMask and returns a new one.
+//
+// The result may alias p, so callers are encouraged to
+// discard p. Not safe for concurrent use.
+func (p pMask) resize(nprocs int32) pMask {
+	maskWords := (nprocs + 31) / 32
+
+	if maskWords <= int32(cap(p)) {
+		return p[:maskWords]
+	}
+	newMask := make([]uint32, maskWords)
+	// No need to copy beyond len, old Ps are irrelevant.
+	copy(newMask, p)
+	return newMask
+}
+
 // pidleput puts p on the _Pidle list. now must be a relatively recent call
 // to nanotime or zero. Returns now or the current time if now was zero.
 //