[llvm-exegesis][NFCI] Counter::Counter(): more useful msg on even open error

[lebedev.ri]

Summary

I'm slowly looking into a new X86 scheduler model,
for AMD Bulldozer CPU, model 2 (bdver2, Piledriver).

And naturally, i have hit that assert :)
I happened to know what it meant, and how to fix it,
but that is not too common knowledge.

[courbet]

Thanks !

[lebedev.ri]

In D47572#1117243, @courbet wrote:

Thanks !

Thank you for the speedy review!

llvm/trunk/tools/llvm-exegesis/lib/PerfHelper.cpp

 Counter::Counter(const PerfEvent &Event) {
   assert(Event.valid());
   const pid_t Pid = 0;    // measure current process/thread.
   const int Cpu = -1;     // measure any processor.
   const int GroupFd = -1; // no grouping of counters.
   const uint32_t Flags = 0;
   perf_event_attr AttrCopy = *Event.attribute();
   FileDescriptor = perf_event_open(&AttrCopy, Pid, Cpu, GroupFd, Flags);
-  assert(FileDescriptor != -1 &&
-         "Unable to open event, make sure your kernel allows user space perf "
-         "monitoring.");
+  if (FileDescriptor == -1) {
+    llvm::errs() << "Unable to open event, make sure your kernel allows user "
+                    "space perf monitoring.\nYou may want to try:\n$ sudo sh "
+                    "-c 'echo -1 > /proc/sys/kernel/perf_event_paranoid'\n";
+  }
+  assert(FileDescriptor != -1 && "Unable to open event");
 }
 
 Counter::~Counter() { close(FileDescriptor); }
 
 void Counter::start() { ioctl(FileDescriptor, PERF_EVENT_IOC_RESET, 0); }
 
 void Counter::stop() { ioctl(FileDescriptor, PERF_EVENT_IOC_DISABLE, 0); }