Re: I.5 - Mmaped count

[Ingo Molnar <mingo@elte.hu>]

> 5/ Mmaped count
>
> It is possible to read counts directly from user space for
> self-monitoring threads. This leverages a HW capability present on
> some processors. On X86, this is possible via RDPMC.
>
> The full 64-bit count is constructed by combining the hardware
> value extracted with an assembly instruction and a base value made
> available thru the mmap. There is an atomic generation count
> available to deal with the race condition.
>
> I believe there is a problem with this approach given that the PMU
> is shared and that events can be multiplexed. That means that even
> though you are self-monitoring, events get replaced on the PMU.
> The assembly instruction is unaware of that, it reads a register
> not an event.
>
> On x86, assume event A is hosted in counter 0, thus you need
> RDPMC(0) to extract the count. But then, the event is replaced by
> another one which reuses counter 0. At the user level, you will
> still use RDPMC(0) but it will read the HW value from a different
> event and combine it with a base count from another one.
>
> To avoid this, you need to pin the event so it stays in the PMU at
> all times. Now, here is something unclear to me. Pinning does not
> mean stay in the SAME register, it means the event stays on the
> PMU but it can possibly change register. To prevent that, I
> believe you need to also set exclusive so that no other group can
> be scheduled, and thus possibly use the same counter.
>
> Looks like this is the only way you can make this actually work.
> Not setting pinned+exclusive, is another pitfall in which many
> people will fall into.

   do {
     seq = pc->lock;

     barrier()
     if (pc->index) {
       count = pmc_read(pc->index - 1);
       count += pc->offset;
     } else
       goto regular_read;

     barrier();
   } while (pc->lock != seq);

We don't see the hole you are referring to. The sequence lock
ensures you get a consistent view.