runtime: ensure heap memstats are updated atomically

For the most part, heap memstats are already updated atomically when
passed down to OS-level memory functions (e.g. sysMap). Elsewhere,
however, they're updated with the heap lock.

In order to facilitate holding the heap lock for less time during
allocation paths, this change more consistently makes the update of
these statistics atomic by calling mSysStat{Inc,Dec} appropriately
instead of simply adding or subtracting. It also ensures these values
are loaded atomically.

Furthermore, an undocumented but safe update condition for these
memstats is during STW, at which point using atomics is unnecessary.
This change also documents this condition in mstats.go.

Updates #35112.

Change-Id: I87d0b6c27b98c88099acd2563ea23f8da1239b66
Reviewed-on: https://go-review.googlesource.com/c/go/+/196638
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgcscavenge.go

@@ -57,6 +57,7 @@ package runtime
 
 import (
 	"math/bits"
+	"runtime/internal/atomic"
 	"unsafe"
 )
 
@@ -78,10 +79,8 @@ const (
 )
 
 // heapRetained returns an estimate of the current heap RSS.
-//
-// mheap_.lock must be held or the world must be stopped.
 func heapRetained() uint64 {
-	return memstats.heap_sys - memstats.heap_released
+	return atomic.Load64(&memstats.heap_sys) - atomic.Load64(&memstats.heap_released)
 }
 
 // gcPaceScavenger updates the scavenger's pacing, particularly
@@ -489,7 +488,7 @@ func (s *pageAlloc) scavengeRangeLocked(ci chunkIdx, base, npages uint) {
 
 	// Update global accounting only when not in test, otherwise
 	// the runtime's accounting will be wrong.
-	memstats.heap_released += uint64(npages) * pageSize
+	mSysStatInc(&memstats.heap_released, uintptr(npages)*pageSize)
 }
 
 // fillAligned returns x but with all zeroes in m-aligned

src/runtime/mheap.go

@@ -1003,7 +1003,7 @@ func (h *mheap) allocManual(npage uintptr, stat *uint64) *mspan {
 		s.limit = s.base() + s.npages<<_PageShift
 		s.state.set(mSpanManual) // Publish the span
 		// Manually managed memory doesn't count toward heap_sys.
-		memstats.heap_sys -= uint64(s.npages << _PageShift)
+		mSysStatDec(&memstats.heap_sys, s.npages*pageSize)
 	}
 
 	// This unlock acts as a release barrier. See mheap.alloc_m.
@@ -1113,7 +1113,7 @@ HaveBase:
 		// sysUsed all the pages that are actually available
 		// in the span.
 		sysUsed(unsafe.Pointer(base), npage*pageSize)
-		memstats.heap_released -= uint64(scav)
+		mSysStatDec(&memstats.heap_released, scav)
 	}
 
 	s := (*mspan)(h.spanalloc.alloc())
@@ -1123,8 +1123,10 @@ HaveBase:
 	}
 	h.setSpans(s.base(), npage, s)
 
-	*stat += uint64(npage << _PageShift)
-	memstats.heap_idle -= uint64(npage << _PageShift)
+	// Update stats.
+	nbytes := npage * pageSize
+	mSysStatInc(stat, nbytes)
+	mSysStatDec(&memstats.heap_idle, nbytes)
 
 	return s
 }
@@ -1172,8 +1174,8 @@ func (h *mheap) grow(npage uintptr) bool {
 		// The allocation is always aligned to the heap arena
 		// size which is always > physPageSize, so its safe to
 		// just add directly to heap_released.
-		memstats.heap_released += uint64(asize)
-		memstats.heap_idle += uint64(asize)
+		mSysStatInc(&memstats.heap_released, asize)
+		mSysStatInc(&memstats.heap_idle, asize)
 
 		// Recalculate nBase
 		nBase = alignUp(h.curArena.base+ask, physPageSize)
@@ -1237,8 +1239,8 @@ func (h *mheap) freeSpan(s *mspan) {
 func (h *mheap) freeManual(s *mspan, stat *uint64) {
 	s.needzero = 1
 	lock(&h.lock)
-	*stat -= uint64(s.npages << _PageShift)
-	memstats.heap_sys += uint64(s.npages << _PageShift)
+	mSysStatDec(stat, s.npages*pageSize)
+	mSysStatInc(&memstats.heap_sys, s.npages*pageSize)
 	h.freeSpanLocked(s, false, true)
 	unlock(&h.lock)
 }
@@ -1264,10 +1266,10 @@ func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool) {
 	}
 
 	if acctinuse {
-		memstats.heap_inuse -= uint64(s.npages << _PageShift)
+		mSysStatDec(&memstats.heap_inuse, s.npages*pageSize)
 	}
 	if acctidle {
-		memstats.heap_idle += uint64(s.npages << _PageShift)
+		mSysStatInc(&memstats.heap_idle, s.npages*pageSize)
 	}
 
 	// Mark the space as free.

src/runtime/mstats.go

@@ -31,7 +31,7 @@ type mstats struct {
 	nfree       uint64 // number of frees
 
 	// Statistics about malloc heap.
-	// Protected by mheap.lock
+	// Updated atomically, or with the world stopped.
 	//
 	// Like MemStats, heap_sys and heap_inuse do not count memory
 	// in manually-managed spans.
@@ -47,15 +47,15 @@ type mstats struct {
 
 	// Statistics about allocation of low-level fixed-size structures.
 	// Protected by FixAlloc locks.
-	stacks_inuse uint64 // bytes in manually-managed stack spans
+	stacks_inuse uint64 // bytes in manually-managed stack spans; updated atomically or during STW
 	stacks_sys   uint64 // only counts newosproc0 stack in mstats; differs from MemStats.StackSys
 	mspan_inuse  uint64 // mspan structures
 	mspan_sys    uint64
 	mcache_inuse uint64 // mcache structures
 	mcache_sys   uint64
 	buckhash_sys uint64 // profiling bucket hash table
-	gc_sys       uint64
-	other_sys    uint64
+	gc_sys       uint64 // updated atomically or during STW
+	other_sys    uint64 // updated atomically or during STW
 
 	// Statistics about garbage collector.
 	// Protected by mheap or stopping the world during GC.