Repository: go
Issue: runtime/pprof: TestLabelSystemstack due to sample with no location
Labels: Testing,NeedsInvestigation

```
#!watchflakes
post <- pkg == "runtime/pprof" && test == "TestLabelSystemstack"
```

This one is a doozy. The failure message comes from here:
https://cs.opensource.google/go/go/+/master:src/runtime/pprof/pprof_test.go;l=1535;drc=18c2033ba587ce63fc9f2d6f52b8bb2e395c561f

That seems to imply that the sample was labeled but its stack was empty(!!), which does seem to be the case in the dump of collected profiles:
```
        1: labels: map[key:[value]]
```

(attn @prattmic, CC @golang/runtime)

```
--- FAIL: TestLabelSystemstack (0.32s)
    pprof_test.go:524: total 85 CPU profile samples collected:
        1: 0x866eef (_ZN6__tsanL14MemoryRangeSetEPNS_11ThreadStateEyyyy.isra.39.part.40:0) 0x8e1ec0 (runtime._System:4432) labels: map[]
        
        2: 0x86a6bc (_ZN6__tsan7MetaMap9FreeRangeEPNS_9ProcessorEyy:0) 0x8e1ec0 (runtime._System:4432) labels: map[key:[value]]
        
        1: 0x8d2f04 (runtime.stdcall1:1090) 0x8d23a9 (runtime.semawakeup:871) 0x8a96ad (runtime.notewakeup:161) 0x8dce48 (runtime.startm:2324) 0x8ded1e (runtime.injectglist.func1:3076 runtime.injectglist:3100) 0x8bf5b6 (runtime.wakeScavenger:222) 0x8bf6d6 (runtime.bgscavenge.func1:268) 0x8f6d21 (runtime.runOneTimer:867) 0x8f6ad2 (runtime.runtimer:775) 0x8df2cf (runtime.checkTimers:3286) 0x8de484 (runtime.stealWork:2868) 0x8dda35 (runtime.findrunnable:2599) 0x8defd8 (runtime.schedule:3187) 0x8df52c (runtime.park_m:3336) 0x905f89 (runtime.mcall:425) labels: map[]
        
        13: 0x907ee6 (runtime.procyield:733) 0x8a9475 (runtime.lock2:69) 0x8bf1f1 (runtime.lockWithRank:22 runtime.lock:36 runtime.setGCPercent.func1:1255) 0x90600d (runtime.systemstack:469) 0x9020a5 (runtime/debug.setGCPercent:1254) 0xa8de99 (runtime/debug.SetGCPercent:92 runtime/pprof.labelHog:1552) 0xa8e152 (runtime/pprof.parallelLabelHog.func1:1565) labels: map[key:[value]]
        
        1: 0x866ef7 (_ZN6__tsanL14MemoryRangeSetEPNS_11ThreadStateEyyyy.isra.39.part.40:0) 0x8e1ec0 (runtime._System:4432) labels: map[]
        
        1: 0x8bf0d0 (runtime.(*gcControllerState).commit:1089) 0x8bf195 (runtime.(*gcControllerState).setGCPercent:1246) 0x8bf204 (runtime.setGCPercent.func1:1256) 0x90600d (runtime.systemstack:469) 0x9020a5 (runtime/debug.setGCPercent:1254) 0xa8de99 (runtime/debug.SetGCPercent:92 runtime/pprof.labelHog:1552) 0xa8e152 (runtime/pprof.parallelLabelHog.func1:1565) labels: map[key:[value]]
        
        5: 0x8a9402 (runtime.lock2:61) 0x8bf1f1 (runtime.lockWithRank:22 runtime.lock:36 runtime.setGCPercent.func1:1255) 0x90600d (runtime.systemstack:469) 0x9020a5 (runtime/debug.setGCPercent:1254) 0xa8de99 (runtime/debug.SetGCPercent:92 runtime/pprof.labelHog:1552) 0xa8e152 (runtime/pprof.parallelLabelHog.func1:1565) labels: map[key:[value]]
        
        27: 0x8d2f84 (runtime.stdcall2:1099) 0x8d204e (runtime.semasleep:819) 0x8a94f7 (runtime.lock2:89) 0x8bf1f1 (runtime.lockWithRank:22 runtime.lock:36 runtime.setGCPercent.func1:1255) 0x90600d (runtime.systemstack:469) labels: map[key:[value]]
        
        4: 0x8a9419 (runtime.lock2:63) 0x8bf1f1 (runtime.lockWithRank:22 runtime.lock:36 runtime.setGCPercent.func1:1255) 0x90600d (runtime.systemstack:469) 0x9020a5 (runtime/debug.setGCPercent:1254) 0xa8de99 (runtime/debug.SetGCPercent:92 runtime/pprof.labelHog:1552) 0xa8e152 (runtime/pprof.parallelLabelHog.func1:1565) labels: map[key:[value]]
        
        11: 0x8e1f00 (runtime._ExternalCode:4433) 0x8e1ec0 (runtime._System:4432) labels: map[key:[value]]
        
        10: 0x8d2f04 (runtime.stdcall1:1090) 0x8d23a9 (runtime.semawakeup:871) 0x8a95f5 (runtime.unlock2:117) 0x8bf238 (runtime.unlockWithRank:31 runtime.unlock:97 runtime.setGCPercent.func1:1259) 0x90600d (runtime.systemstack:469) labels: map[key:[value]]
        
        1: 0x8d3004 (runtime.stdcall3:1108) 0x8b5ba4 (runtime.sysUnused:33) 0x8c098a (runtime.(*pageAlloc).scavengeRangeLocked:775) 0x8c07cd (runtime.(*pageAlloc).scavengeOneFast:726) 0x8c0324 (runtime.(*pageAlloc).scavengeOne:637) 0x8bfcfc (runtime.(*pageAlloc).scavenge.func1:454) 0x90600d (runtime.systemstack:469) labels: map[]
        
        3: 0x907ee4 (runtime.procyield:732) 0x8a9475 (runtime.lock2:69) 0x8bf1f1 (runtime.lockWithRank:22 runtime.lock:36 runtime.setGCPercent.func1:1255) 0x90600d (runtime.systemstack:469) 0x9020a5 (runtime/debug.setGCPercent:1254) 0xa8de99 (runtime/debug.SetGCPercent:92 runtime/pprof.labelHog:1552) 0xa8e152 (runtime/pprof.parallelLabelHog.func1:1565) labels: map[key:[value]]
        
        1: 0x8d2037 (runtime.semasleep:819) 0x8a94f7 (runtime.lock2:89) 0x8bf1f1 (runtime.lockWithRank:22 runtime.lock:36 runtime.setGCPercent.func1:1255) 0x90600d (runtime.systemstack:469) 0x9020a5 (runtime/debug.setGCPercent:1254) 0xa8de99 (runtime/debug.SetGCPercent:92 runtime/pprof.labelHog:1552) 0xa8e152 (runtime/pprof.parallelLabelHog.func1:1565) labels: map[key:[value]]
        
        1: 0xa8de9a (runtime/pprof.labelHog:1549) 0xa8e152 (runtime/pprof.parallelLabelHog.func1:1565) labels: map[key:[value]]
        
        1: 0x8d2f04 (runtime.stdcall1:1090) 0x8d23a9 (runtime.semawakeup:871) 0x8a95f5 (runtime.unlock2:117) 0x8c07e7 (runtime.unlockWithRank:31 runtime.unlock:97 runtime.(*pageAlloc).scavengeOneFast:727) 0x8c0324 (runtime.(*pageAlloc).scavengeOne:637) 0x8bfcfc (runtime.(*pageAlloc).scavenge.func1:454) 0x90600d (runtime.systemstack:469) labels: map[]
        
        1: labels: map[key:[value]]
        
        1: 0x89a3a8 (.text$_ZN6__tsan14DenseSlabAllocINS_10ClockBlockELy65536ELy1024EE6RefillEPNS_19DenseSlabAllocCacheE:0) 0x8e1ec0 (runtime._System:4432) labels: map[]
        
    pprof_test.go:595: runtime.systemstack;key=value: 64
    pprof_test.go:1535: Sample labeled got true want false: 
FAIL
FAIL	runtime/pprof	19.664s
```

`greplogs --dashboard -md -l -e '(?ms)FAIL: TestLabelSystemstack.*Sample labeled got true want false:\s*$'`

[2022-03-05T08:36:13-55a60ca/windows-amd64-race](https://build.golang.org/log/daf96d123ec840e876eb2865886e67013f1161da)