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From:   Atish Patra <atishp@atishpatra.org>
Date:   Wed, 19 Jul 2023 10:14:36 -0700
Message-ID: <CAOnJCUKCx0Jh1igZytKAYePhhUBt=x5ANCVx4FRTbLtxGkyowA@mail.gmail.com>
Subject: Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters
To:     =?UTF-8?Q?R=C3=A9mi_Denis=2DCourmont?= <remi@remlab.net>
Cc:     linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        Aurelien Jarno <aurelien@aurel32.net>,
        Mathieu Malaterre <malat@debian.org>,
        Jan Newger <jannewger@google.com>
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On Wed, Jul 19, 2023 at 7:46=E2=80=AFAM R=C3=A9mi Denis-Courmont <remi@reml=
ab.net> wrote:
>
> Le keskiviikkona 19. hein=C3=A4kuuta 2023, 1.48.49 EEST Atish Patra a =C3=
=A9crit :
> > > Isn't RDTIM susceptible to interference from power management and CPU
> > > frequency scaling? I suppose that RDCYCLE may behave differently depe=
nding
> > > on PM in *some* designs, but that would still be way better than RDTI=
ME
> > > for the purpose.
> >
> > Yes. But that's what it is probably using for other ISAs ?
>
> At least on AArch64, it is using either Linux perf cycle counter, or if t=
hat
> is disabled at build time, the raw PMU cycle counter - which obviously le=
ads
> to SIGILL on Linux, just like this MR would do with RDCYCLE.
>

Good to know. Thanks for the clarification.

> Again, I do not *personally* have objections to disabling RDCYCLE for
> userspace (somebody else does, but that's neither my nor your problem). I=
 do
> have objections to the wording of some of the commit messages though.
>

Completely agreed. We will update the commit text with more clarification i=
n v5.

> > My point was it should just do whatever it does for other ISA. RISC-V i=
s no
> > special in that regard.
>
> Sure. My point is that RDTIME may be great for, so to say, system-level
> benchmarks. For FFmpeg that could something like how long it takes to
> transcode a video. But it doesn't seem to make much sense for
> microbenchmarking of single threaded tightly optimised loops, as opposed =
to
> RDCYCLE (or a wrapper for RDCYCLE).
>
> --
> R=C3=A9mi Denis-Courmont
> http://www.remlab.net/
>
>
>


--=20
Regards,
Atish