runtime: fix formula for computing number of padding bytes

In order to prevent false sharing of cache lines, structs are
padded with some number of bytes. These bytes are unused, serving
only to make the size of the struct a multiple of the size of the
cache line.

The current calculation of how much to pad is an overestimation,
when the struct size is already a multiple of the cache line size
without padding. For these cases, no padding is necessary, and
the size of the inner pad field should be 0. The bug is that the
pad field is sized to a whole 'nother cache line, wasting space.

Here is the current formula that can never return 0:
cpu.CacheLinePadSize - unsafe.Sizeof(myStruct{})%cpu.CacheLinePadSize

This change simply mods that calculation by cpu.CacheLinePadSize,
so that 0 will be returned instead of cpu.CacheLinePadSize.

Change-Id: I26a2b287171bf47a3b9121873b2722f728381b5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/414214
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/mheap.go

@@ -200,7 +200,7 @@ type mheap struct {
 	// central is indexed by spanClass.
 	central [numSpanClasses]struct {
 		mcentral mcentral
-		pad      [cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte
+		pad      [(cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize) % cpu.CacheLinePadSize]byte
 	}
 
 	spanalloc             fixalloc // allocator for span*

src/runtime/stack.go

@@ -157,7 +157,7 @@ const (
 // There is a free list for each order.
 var stackpool [_NumStackOrders]struct {
 	item stackpoolItem
-	_    [cpu.CacheLinePadSize - unsafe.Sizeof(stackpoolItem{})%cpu.CacheLinePadSize]byte
+	_    [(cpu.CacheLinePadSize - unsafe.Sizeof(stackpoolItem{})%cpu.CacheLinePadSize) % cpu.CacheLinePadSize]byte
 }
 
 type stackpoolItem struct {