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runtime: put ReadMemStats debug assertions behind a double-check mode

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ReadMemStats has a few assertions it makes about the consistency of the
stats it's about to produce. Specifically, how those stats line up with
runtime-internal stats. These checks are generally useful, but crashing
just because some stats are wrong is a heavy price to pay.

For a long time this wasn't a problem, but very recently it became a
real problem. It turns out that there's real benign skew that can happen
wherein sysmon (which doesn't synchronize with a STW) generates a trace
event when tracing is enabled, and may mutate some stats while
ReadMemStats is running its checks.

Fix this by synchronizing with both sysmon and the tracer. This is a bit
heavy-handed, but better that than false positives.

Also, put the checks behind a debug mode. We want to reduce the risk of
backporting this change, and again, it's not great to crash just because
user-facing stats are off. Still, enable this debug mode during the
runtime tests so we don't lose quite as much coverage from disabling
these checks by default.

Fixes #64401.

Change-Id: I9adb3e5c7161d207648d07373a11da8a5f0fda9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/545277
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
This commit is contained in:
Michael Anthony Knyszek 2023-11-27 22:27:32 +00:00 committed by Michael Knyszek
parent 5df4a6376f
commit b2efd1de97
3 changed files with 71 additions and 50 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
src/runtime/export_test.go
View File

@ -464,6 +464,8 @@ func ReadMetricsSlow(memStats *MemStats, samplesp unsafe.Pointer, len, cap int)
startTheWorld(stw)
}
var DoubleCheckReadMemStats = &doubleCheckReadMemStats
// ReadMemStatsSlow returns both the runtime-computed MemStats and
// MemStats accumulated by scanning the heap.
func ReadMemStatsSlow() (base, slow MemStats) {

5
src/runtime/gc_test.go
View File

@ -577,6 +577,11 @@ func TestPageAccounting(t *testing.T) {
}
}
func init() {
// Enable ReadMemStats' double-check mode.
*runtime.DoubleCheckReadMemStats = true
}
func TestReadMemStats(t *testing.T) {
base, slow := runtime.ReadMemStatsSlow()
if base != slow {

114
src/runtime/mstats.go
View File

@ -361,6 +361,11 @@ func ReadMemStats(m *MemStats) {
startTheWorld(stw)
}
// doubleCheckReadMemStats controls a double-check mode for ReadMemStats that
// ensures consistency between the values that ReadMemStats is using and the
// runtime-internal stats.
var doubleCheckReadMemStats = false
// readmemstats_m populates stats for internal runtime values.
//
// The world must be stopped.
@ -435,56 +440,65 @@ func readmemstats_m(stats *MemStats) {
heapGoal := gcController.heapGoal()
// The world is stopped, so the consistent stats (after aggregation)
// should be identical to some combination of memstats. In particular:
//
// * memstats.heapInUse == inHeap
// * memstats.heapReleased == released
// * memstats.heapInUse + memstats.heapFree == committed - inStacks - inWorkBufs - inPtrScalarBits
// * memstats.totalAlloc == totalAlloc
// * memstats.totalFree == totalFree
//
// Check if that's actually true.
//
// TODO(mknyszek): Maybe don't throw here. It would be bad if a
// bug in otherwise benign accounting caused the whole application
// to crash.
if gcController.heapInUse.load() != uint64(consStats.inHeap) {
print("runtime: heapInUse=", gcController.heapInUse.load(), "\n")
print("runtime: consistent value=", consStats.inHeap, "\n")
throw("heapInUse and consistent stats are not equal")
}
if gcController.heapReleased.load() != uint64(consStats.released) {
print("runtime: heapReleased=", gcController.heapReleased.load(), "\n")
print("runtime: consistent value=", consStats.released, "\n")
throw("heapReleased and consistent stats are not equal")
}
heapRetained := gcController.heapInUse.load() + gcController.heapFree.load()
consRetained := uint64(consStats.committed - consStats.inStacks - consStats.inWorkBufs - consStats.inPtrScalarBits)
if heapRetained != consRetained {
print("runtime: global value=", heapRetained, "\n")
print("runtime: consistent value=", consRetained, "\n")
throw("measures of the retained heap are not equal")
}
if gcController.totalAlloc.Load() != totalAlloc {
print("runtime: totalAlloc=", gcController.totalAlloc.Load(), "\n")
print("runtime: consistent value=", totalAlloc, "\n")
throw("totalAlloc and consistent stats are not equal")
}
if gcController.totalFree.Load() != totalFree {
print("runtime: totalFree=", gcController.totalFree.Load(), "\n")
print("runtime: consistent value=", totalFree, "\n")
throw("totalFree and consistent stats are not equal")
}
// Also check that mappedReady lines up with totalMapped - released.
// This isn't really the same type of "make sure consistent stats line up" situation,
// but this is an opportune time to check.
if gcController.mappedReady.Load() != totalMapped-uint64(consStats.released) {
print("runtime: mappedReady=", gcController.mappedReady.Load(), "\n")
print("runtime: totalMapped=", totalMapped, "\n")
print("runtime: released=", uint64(consStats.released), "\n")
print("runtime: totalMapped-released=", totalMapped-uint64(consStats.released), "\n")
throw("mappedReady and other memstats are not equal")
if doubleCheckReadMemStats {
// Only check this if we're debugging. It would be bad to crash an application
// just because the debugging stats are wrong. We mostly rely on tests to catch
// these issues, and we enable the double check mode for tests.
//
// The world is stopped, so the consistent stats (after aggregation)
// should be identical to some combination of memstats. In particular:
//
// * memstats.heapInUse == inHeap
// * memstats.heapReleased == released
// * memstats.heapInUse + memstats.heapFree == committed - inStacks - inWorkBufs - inPtrScalarBits
// * memstats.totalAlloc == totalAlloc
// * memstats.totalFree == totalFree
//
// Check if that's actually true.
//
// Prevent sysmon and the tracer from skewing the stats since they can
// act without synchronizing with a STW. See #64401.
lock(&sched.sysmonlock)
lock(&trace.lock)
if gcController.heapInUse.load() != uint64(consStats.inHeap) {
print("runtime: heapInUse=", gcController.heapInUse.load(), "\n")
print("runtime: consistent value=", consStats.inHeap, "\n")
throw("heapInUse and consistent stats are not equal")
}
if gcController.heapReleased.load() != uint64(consStats.released) {
print("runtime: heapReleased=", gcController.heapReleased.load(), "\n")
print("runtime: consistent value=", consStats.released, "\n")
throw("heapReleased and consistent stats are not equal")
}
heapRetained := gcController.heapInUse.load() + gcController.heapFree.load()
consRetained := uint64(consStats.committed - consStats.inStacks - consStats.inWorkBufs - consStats.inPtrScalarBits)
if heapRetained != consRetained {
print("runtime: global value=", heapRetained, "\n")
print("runtime: consistent value=", consRetained, "\n")
throw("measures of the retained heap are not equal")
}
if gcController.totalAlloc.Load() != totalAlloc {
print("runtime: totalAlloc=", gcController.totalAlloc.Load(), "\n")
print("runtime: consistent value=", totalAlloc, "\n")
throw("totalAlloc and consistent stats are not equal")
}
if gcController.totalFree.Load() != totalFree {
print("runtime: totalFree=", gcController.totalFree.Load(), "\n")
print("runtime: consistent value=", totalFree, "\n")
throw("totalFree and consistent stats are not equal")
}
// Also check that mappedReady lines up with totalMapped - released.
// This isn't really the same type of "make sure consistent stats line up" situation,
// but this is an opportune time to check.
if gcController.mappedReady.Load() != totalMapped-uint64(consStats.released) {
print("runtime: mappedReady=", gcController.mappedReady.Load(), "\n")
print("runtime: totalMapped=", totalMapped, "\n")
print("runtime: released=", uint64(consStats.released), "\n")
print("runtime: totalMapped-released=", totalMapped-uint64(consStats.released), "\n")
throw("mappedReady and other memstats are not equal")
}
unlock(&trace.lock)
unlock(&sched.sysmonlock)
}
// We've calculated all the values we need. Now, populate stats.