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Message-ID: <YuuIBr2aqB+1992Q@gmail.com>
Date: Thu, 4 Aug 2022 10:49:10 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Vince Weaver <vincent.weaver@...ne.edu>
Cc: Ian Rogers <irogers@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Kajol Jain <kjain@...ux.ibm.com>,
Andi Kleen <ak@...ux.intel.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Anshuman Khandual <anshuman.khandual@....com>,
linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
Rob Herring <robh@...nel.org>,
Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v3 1/3] perf: Align user space counter reading with code
* Vince Weaver <vincent.weaver@...ne.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@...gle.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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