Re: I.5 - Mmaped count

Peter Zijlstra
2009-06-22T12:35:59Z

On Mon, 2009-06-22 at 14:25 +0200, stephane eranian wrote:
> On Mon, Jun 22, 2009 at 1:52 PM, Ingo Molnar<mingo@elte.hu> wrote:
> >> 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.
> >
> Let's take an example, with two groups, one event in each group.
> Both events scheduled on counter0, i.e,, rdpmc(0). The 2 groups
> are multiplexed, one each tick. The user gets 2 file descriptors
> and thus two mmap'ed pages.
>
> Suppose the user wants to read, using the above loop, the value of the
> event in the first group BUT it's the 2nd group that is currently active
> and loaded on counter0, i.e., rdpmc(0) returns the value of the 2nd event.
>
> Unless you tell me that pc->index is marked invalid (0) when the
> event is not scheduled. I don't see how you can avoid reading
> the wrong value. I am assuming that is the event is not scheduled
> lock remains constant.

Indeed, pc->index == 0 means its not currently available.

> Assuming the event is active when you enter the loop and you
> read a value. How to get the timing information to scale the
> count?

I think we would have to add that do the data page,.. something like the
below?

Paulus?

---
Index: linux-2.6/include/linux/perf_counter.h
===================================================================
--- linux-2.6.orig/include/linux/perf_counter.h
+++ linux-2.6/include/linux/perf_counter.h
@@ -232,6 +232,10 @@ struct perf_counter_mmap_page {
__u32 lock; /* seqlock for synchronization */
__u32 index; /* hardware counter identifier */
__s64 offset; /* add to hardware counter value */
+ __u64 total_time; /* total time counter active */
+ __u64 running_time; /* time counter on cpu */
+
+ __u64 __reserved[123]; /* align at 1k */

/*
* Control data for the mmap() data buffer.
Index: linux-2.6/kernel/perf_counter.c
===================================================================
--- linux-2.6.orig/kernel/perf_counter.c
+++ linux-2.6/kernel/perf_counter.c
@@ -1782,6 +1782,12 @@ void perf_counter_update_userpage(struct
if (counter->state == PERF_COUNTER_STATE_ACTIVE)
userpg->offset -= atomic64_read(&counter->hw.prev_count);

+ userpg->total_time = counter->total_time_enabled +
+ atomic64_read(&counter->child_total_time_enabled);
+
+ userpg->running_time = counter->total_time_running +
+ atomic64_read(&counter->child_total_time_running);
+
barrier();
++userpg->lock;
preempt_enable();