Re: [PATCH v3 1/3] perf: Align user space counter reading with code

[Vince Weaver <vincent.weaver@maine.edu>]

On Tue, 19 Jul 2022, Ian Rogers wrote:

> Align the user space counter reading documentation with the code in
> perf_mmap__read_self. Previously the documentation was based on the perf
> rdpmc test, but now general purpose code is provided by libperf.
> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  include/uapi/linux/perf_event.h       | 35 +++++++++++++++++----------
>  tools/include/uapi/linux/perf_event.h | 35 +++++++++++++++++----------
>  2 files changed, 44 insertions(+), 26 deletions(-)
> 
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index d37629dbad72..6826dabb7e03 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -538,9 +538,13 @@ struct perf_event_mmap_page {
>  	 *
>  	 *     if (pc->cap_usr_time && enabled != running) {
>  	 *       cyc = rdtsc();
> -	 *       time_offset = pc->time_offset;
>  	 *       time_mult   = pc->time_mult;
>  	 *       time_shift  = pc->time_shift;
> +	 *       time_offset = pc->time_offset;
> +	 *       if (pc->cap_user_time_short) {
> +	 *         time_cycles = pc->time_cycles;
> +	 *         time_mask = pc->time_mask;
> +	 *       }

From what I've been told, and from what perf_mmap__read_self() actually 
does, many of these MMAP fields need to be accessed by READ_ONCE()
(a GPLv2 only interface) to be correct.

Should we update perf_event.h to reflect this?  Otherwise it's confusing 
when the actual code and the documentation in the header don't match like 
this.  As an example, see the actual code snippets from
perf_mmap__read_self()

		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);

...


Vince