Messages in this thread | | | Date | Thu, 4 Aug 2022 10:49:10 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v3 1/3] perf: Align user space counter reading with code |
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* Vince Weaver <vincent.weaver@maine.edu> wrote:
> 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.
BTW., for this purpose I guess we could add a READ_ONCE() variant to perf tooling that reimplements it with more relaxed licensing, so that the headers & sample code can be included in GPL-incompatible projects?
READ_ONCE() isn't a super complicated primitive, and we've always been permissive with simple primitives.
Thanks,
Ingo
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