Apparent regression after JDK-8204552 fix.
Amount of threads reported incorrectly for summary and detail diff when used by jcmd. During comparison to baseline threads amount is always reported as 0 on non-AIX platforms:
$ jcmd <PID> VM.native_memory baseline
$ jcmd <PID> VM.native_memory summary.diff
...
Thread (reserved=23622KB +1027KB, committed=1358KB +107KB)
              (thread #0)
              (stack: reserved=23552KB +1024KB, committed=1288KB +104KB)
              (malloc=45KB +2KB #142 +6)
              (arena=25KB +1d #44 +2)
...
This is happening because ThreadStackTracker::track_as_vm() condition is ignored during MallocMemorySnapshot collection and thread count is always reported as malloc count of allocations for mtThreadStack:
inline size_t thread_count() const {
 MallocMemorySnapshot* s = const_cast<MallocMemorySnapshot*>(this);
 return s->by_type(mtThreadStack)->malloc_count();
}
MallocMemorySnapshot::thread_count() should consider ThreadStackTracker::track_as_vm().
Issue is not reproducible if ThreadStackTracker::track_as_vm() == false.