[vm] Don't enable the profiler by default. Enable the profiler at startup with --observe, or later …

runtime/vm/profiler.cc

Index: runtime/vm/profiler.cc
diff --git a/runtime/vm/profiler.cc b/runtime/vm/profiler.cc
index a99b48ee84b38090e31fdc1e74dc7b6e2eef88a3..799e4a7f0f9f03627b8ed7cac59c4f13ff8ad869 100644
--- a/runtime/vm/profiler.cc
+++ b/runtime/vm/profiler.cc
@@ -69,6 +69,7 @@ void Profiler::InitOnce() {
   SetSamplePeriod(FLAG_profile_period);
   SetSampleDepth(FLAG_max_profile_depth);
   Sample::InitOnce();
+#if !defined(TARGET_ARCH_DBC)

[zra, 2017/07/31 15:03:11]

I thought the profiler was kind of working somewhat for DBC. Does this CL turn
it off?

[rmacnak, 2017/07/31 21:12:12]

Removed; this was a bad merge. I was working from a CL I started a long time ago
before the profiler support was added for DBC. The profiler indeed works for
DBC, though it only has function-level resolution instead of instruction-level
resolution since we avoid flushing the instruction pointer to memory the
profiler can read.

   if (!FLAG_profiler) {
     return;
   }
@@ -78,9 +79,11 @@ void Profiler::InitOnce() {
   // Zero counters.
   memset(&counters_, 0, sizeof(counters_));
   NativeSymbolResolver::InitOnce();
+  ThreadInterrupter::InitOnce();
   ThreadInterrupter::SetInterruptPeriod(FLAG_profile_period);
   ThreadInterrupter::Startup();
   initialized_ = true;
+#endif  // !DBC
 }
 
 void Profiler::InitAllocationSampleBuffer() {
@@ -91,12 +94,16 @@ void Profiler::InitAllocationSampleBuffer() {
 }
 
 void Profiler::Shutdown() {
+#if !defined(TARGET_ARCH_DBC)
   if (!FLAG_profiler) {
     return;
   }
   ASSERT(initialized_);
   ThreadInterrupter::Shutdown();
   NativeSymbolResolver::ShutdownOnce();
+// Note we do not free the sample buffer because there may be SIGPROFs

[zra, 2017/07/31 15:03:11]

Can you set a "shutting down" bit somewhere that the signal handler will look
at? If it's set, then it can e.g. return immediately without touching the sample
buffer.

[rmacnak, 2017/07/31 21:12:12]

The signal handler could be between that check and writing the sample when we
want to free the sample buffer. We'd need a critical section around both the
check and the use of the sample buffer (which is nearly all of the signal
handler). thread_interrupt.cc has a lengthy comment explaining that we can't
take locks.

[zra, 2017/07/31 21:59:28]

Maybe some thing like:

Signal handler:
AtomicInc(&some_counter);
if (shutting_down) {
  AtomicDec(&some_counter);
  return;
}
...
AtomicDec(&some_counter);


Thread Interrupter thread:
RemoveSignalHandler();
shutting_down = true;
while (some_counter > 0) {
  usleep(1000);
}
free(sample_buffer);

[rmacnak, 2017/08/01 19:58:58]

Added TODO(30309)

+// in flight.
+#endif  // !DBC
 }
 
 void Profiler::SetSampleDepth(intptr_t depth) {