runtime: convert timeHistogram to atomic types

I've dropped the note that sched.timeToRun is protected by sched.lock,
as it does not seem to be true.

For #53821.

Change-Id: I03f8dc6ca0bcd4ccf3ec113010a0aa39c6f7d6ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/419449
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>

7 files changed, 11 insertions, 29 deletions

diff --git a/src/runtime/align_runtime_test.go b/src/runtime/align_runtime_test.go
index e7af4cd6ff..2c448d4a09 100644
--- a/src/runtime/align_runtime_test.go
+++ b/src/runtime/align_runtime_test.go
@@ -17,8 +17,6 @@ var AtomicFields = []uintptr{
 	unsafe.Offsetof(p{}.timer0When),
 	unsafe.Offsetof(p{}.timerModifiedEarliest),
 	unsafe.Offsetof(p{}.gcFractionalMarkTime),
-	unsafe.Offsetof(schedt{}.timeToRun),
-	unsafe.Offsetof(timeHistogram{}.underflow),
 	unsafe.Offsetof(profBuf{}.overflow),
 	unsafe.Offsetof(profBuf{}.overflowTime),
 	unsafe.Offsetof(heapStatsDelta{}.tinyAllocCount),
@@ -37,10 +35,8 @@ var AtomicFields = []uintptr{
 	unsafe.Offsetof(lfnode{}.next),
 	unsafe.Offsetof(mstats{}.last_gc_nanotime),
 	unsafe.Offsetof(mstats{}.last_gc_unix),
-	unsafe.Offsetof(mstats{}.gcPauseDist),
 	unsafe.Offsetof(ticksType{}.val),
 	unsafe.Offsetof(workType{}.bytesMarked),
-	unsafe.Offsetof(timeHistogram{}.counts),
 }
 
 // AtomicVariables is the set of global variables on which we perform
diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 81f60b3ada..d9f36c06c2 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -1244,9 +1244,9 @@ func (th *TimeHistogram) Count(bucket, subBucket uint) (uint64, bool) {
 	t := (*timeHistogram)(th)
 	i := bucket*TimeHistNumSubBuckets + subBucket
 	if i >= uint(len(t.counts)) {
-		return t.underflow, false
+		return t.underflow.Load(), false
 	}
-	return t.counts[i], true
+	return t.counts[i].Load(), true
 }
 
 func (th *TimeHistogram) Record(duration int64) {
diff --git a/src/runtime/histogram.go b/src/runtime/histogram.go
index eddfbab3bc..d2e6367c84 100644
--- a/src/runtime/histogram.go
+++ b/src/runtime/histogram.go
@@ -66,18 +66,16 @@ const (
 // It is an HDR histogram with exponentially-distributed
 // buckets and linearly distributed sub-buckets.
 //
-// Counts in the histogram are updated atomically, so it is safe
-// for concurrent use. It is also safe to read all the values
-// atomically.
+// The histogram is safe for concurrent reads and writes.
 type timeHistogram struct {
-	counts [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64
+	counts [timeHistNumSuperBuckets * timeHistNumSubBuckets]atomic.Uint64
 
 	// underflow counts all the times we got a negative duration
 	// sample. Because of how time works on some platforms, it's
 	// possible to measure negative durations. We could ignore them,
 	// but we record them anyway because it's better to have some
 	// signal that it's happening than just missing samples.
-	underflow uint64
+	underflow atomic.Uint64
 }
 
 // record adds the given duration to the distribution.
@@ -88,7 +86,7 @@ type timeHistogram struct {
 //go:nosplit
 func (h *timeHistogram) record(duration int64) {
 	if duration < 0 {
-		atomic.Xadd64(&h.underflow, 1)
+		h.underflow.Add(1)
 		return
 	}
 	// The index of the exponential bucket is just the index
@@ -116,7 +114,7 @@ func (h *timeHistogram) record(duration int64) {
 	} else {
 		subBucket = uint(duration)
 	}
-	atomic.Xadd64(&h.counts[superBucket*timeHistNumSubBuckets+subBucket], 1)
+	h.counts[superBucket*timeHistNumSubBuckets+subBucket].Add(1)
 }
 
 const (
diff --git a/src/runtime/metrics.go b/src/runtime/metrics.go
index 986121b9c2..313850a3a0 100644
--- a/src/runtime/metrics.go
+++ b/src/runtime/metrics.go
@@ -7,7 +7,6 @@ package runtime
 // Metrics implementation exported to runtime/metrics.
 
 import (
-	"runtime/internal/atomic"
 	"unsafe"
 )
 
@@ -197,9 +196,9 @@ func initMetrics() {
 				// The bottom-most bucket, containing negative values, is tracked
 				// as a separately as underflow, so fill that in manually and then
 				// iterate over the rest.
-				hist.counts[0] = atomic.Load64(&memstats.gcPauseDist.underflow)
+				hist.counts[0] = memstats.gcPauseDist.underflow.Load()
 				for i := range memstats.gcPauseDist.counts {
-					hist.counts[i+1] = atomic.Load64(&memstats.gcPauseDist.counts[i])
+					hist.counts[i+1] = memstats.gcPauseDist.counts[i].Load()
 				}
 			},
 		},
@@ -327,9 +326,9 @@ func initMetrics() {
 		"/sched/latencies:seconds": {
 			compute: func(_ *statAggregate, out *metricValue) {
 				hist := out.float64HistOrInit(timeHistBuckets)
-				hist.counts[0] = atomic.Load64(&sched.timeToRun.underflow)
+				hist.counts[0] = sched.timeToRun.underflow.Load()
 				for i := range sched.timeToRun.counts {
-					hist.counts[i+1] = atomic.Load64(&sched.timeToRun.counts[i])
+					hist.counts[i+1] = sched.timeToRun.counts[i].Load()
 				}
 			},
 		},
diff --git a/src/runtime/mstats.go b/src/runtime/mstats.go
index 0029ea956c..458350da02 100644
--- a/src/runtime/mstats.go
+++ b/src/runtime/mstats.go
@@ -334,10 +334,6 @@ func init() {
 		println(offset)
 		throw("memstats.heapStats not aligned to 8 bytes")
 	}
-	if offset := unsafe.Offsetof(memstats.gcPauseDist); offset%8 != 0 {
-		println(offset)
-		throw("memstats.gcPauseDist not aligned to 8 bytes")
-	}
 	// Ensure the size of heapStatsDelta causes adjacent fields/slots (e.g.
 	// [3]heapStatsDelta) to be 8-byte aligned.
 	if size := unsafe.Sizeof(heapStatsDelta{}); size%8 != 0 {
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index a2a02ebf9a..c3144b4dde 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -703,11 +703,6 @@ func schedinit() {
 	sigsave(&gp.m.sigmask)
 	initSigmask = gp.m.sigmask
 
-	if offset := unsafe.Offsetof(sched.timeToRun); offset%8 != 0 {
-		println(offset)
-		throw("sched.timeToRun not aligned to 8 bytes")
-	}
-
 	goargs()
 	goenvs()
 	parsedebugvars()
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 9216765fc6..e706cf7354 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -843,8 +843,6 @@ type schedt struct {
 	// timeToRun is a distribution of scheduling latencies, defined
 	// as the sum of time a G spends in the _Grunnable state before
 	// it transitions to _Grunning.
-	//
-	// timeToRun is protected by sched.lock.
 	timeToRun timeHistogram
 }