Messages in this thread | | | From | Vince Weaver <> | Date | Wed, 20 Jul 2022 11:06:08 -0400 (EDT) | Subject | Re: [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
| |