Re: [PATCH v2 5/9] libperf: Add support for user space counter access

[Ian Rogers <irogers@google.com>]

On Wed, Sep 2, 2020 at 12:48 PM Rob Herring <robh@kernel.org> wrote:
>
> On Wed, Sep 2, 2020 at 12:07 PM Ian Rogers <irogers@google.com> wrote:
> >
> > On Fri, Aug 28, 2020 at 1:56 PM Rob Herring <robh@kernel.org> wrote:
> > >
> > > x86 and arm64 can both support direct access of event counters in
> > > userspace. The access sequence is less than trivial and currently exists
> > > in perf test code (tools/perf/arch/x86/tests/rdpmc.c) with copies in
> > > projects such as PAPI and libpfm4.
> > >
> > > In order to support usersapce access, an event must be mmapped. While
> > > there's already mmap support for evlist, the usecase is a bit different
> > > than the self monitoring with userspace access. So let's add a new
> > > perf_evsel__mmap() function to mmap an evsel. This allows implementing
> > > userspace access as a fastpath for perf_evsel__read().
> > >
> > > The mmapped address is returned by perf_evsel__mmap() primarily for
> > > users/tests to check if userspace access is enabled.
> > >
> > > Signed-off-by: Rob Herring <robh@kernel.org>
> > > ---
>
> > > +int perf_mmap__read_self(struct perf_mmap *map, struct perf_counts_values *count)
> > > +{
> > > +       struct perf_event_mmap_page *pc = map->base;
> > > +       u32 seq, idx, time_mult = 0, time_shift = 0;
> > > +       u64 cnt, cyc = 0, time_offset = 0, time_cycles = 0, time_mask = ~0ULL;
> > > +
> > > +       BUG_ON(!pc);
> > > +
> > > +       if (!pc->cap_user_rdpmc)
> > > +               return -1;
> > > +
> > > +       do {
> > > +               seq = READ_ONCE(pc->lock);
> > > +               barrier();
> > > +
> > > +               count->ena = READ_ONCE(pc->time_enabled);
> > > +               count->run = READ_ONCE(pc->time_running);
> > > +
> > > +               if (pc->cap_user_time && count->ena != count->run) {
> > > +                       cyc = read_timestamp();
> > > +                       time_mult = READ_ONCE(pc->time_mult);
> > > +                       time_shift = READ_ONCE(pc->time_shift);
> > > +                       time_offset = READ_ONCE(pc->time_offset);
> > > +
> > > +                       if (pc->cap_user_time_short) {
> > > +                               time_cycles = READ_ONCE(pc->time_cycles);
> > > +                               time_mask = READ_ONCE(pc->time_mask);
> > > +                       }
> > > +               }
> > > +
> > > +               idx = READ_ONCE(pc->index);
> > > +               cnt = READ_ONCE(pc->offset);
> > > +               if (pc->cap_user_rdpmc && idx) {
> > > +                       u64 evcnt = read_perf_counter(idx - 1);
> > > +                       u16 width = READ_ONCE(pc->pmc_width);
> > > +
> > > +                       evcnt <<= 64 - width;
> > > +                       evcnt >>= 64 - width;
> > > +                       cnt += evcnt;
> > > +               } else
> > > +                       return -1;
> > > +
> > > +               barrier();
> > > +       } while (READ_ONCE(pc->lock) != seq);
> > > +
> > > +       if (count->ena != count->run) {
> >
> > There's an existing bug here that I tried to resolve in this patch:
> > https://lore.kernel.org/lkml/CAP-5=fVRdqvswtyQMg5cB+ntTGda+SAYskjTQednEH-AeZo13g@mail.gmail.com/
> > Due to multiplexing, enabled may be > 0 but run == 0 and the divide
> > below can end up with divide by zero.
>
> Yeah, I saw that, but didn't try to also fix that issue here.
>
> > I like the idea of this code being in a library, there's an intent
> > that the perf_event.h and test code be copy-paste-able, but there is
> > some pre-existing divergence. It would be nice if this code could be
> > closer to the sample code in both the test and perf_event.h.
>
> The only way we get and keep all the versions of the code aligned is
> removing the other copies. We should just remove the code comment from
> perf_event.h IMO. If rdpmc.c is going to stick around given some
> resistance to removing it, then perhaps it should be converted to use
> libperf. At that point it could also be arch independent. Though I
> don't like the idea of having the same test twice.

This makes sense to me, perhaps others could comment. Given the
cleaned up API fixing or deleting tools/perf/arch/x86/tests/rdpmc.c is
desirable (as your patch set does). I wondered if we could do Jiri's
suggestion to run the lib/perf tests with perf test. One way would be
to have shell script wrapper in tools/perf/tests/shell. It's not clear
how to make a dependency from a shell script there and tests built
elsewhere in the tree though.

> > As per the change above, I think running and enabled times need to be
> > out arguments.
>
> They are now in this version.

Sorry, my mistake. I'd missed that.

Thanks,
Ian

> Rob