lkml.org 
[lkml]   [2022]   [Jul]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH v3 1/3] perf: Align user space counter reading with code
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

\
 
 \ /
  Last update: 2022-07-20 17:07    [W:0.078 / U:0.224 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site