linux-kernel - Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters

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Message-ID: <90c8509f-bd0a-4996-adca-86f6116b5129@ghiti.fr>
Date:   Thu, 20 Jul 2023 11:13:36 +0200
From:   Alexandre Ghiti <alex@...ti.fr>
To:     Atish Patra <atishp@...shpatra.org>,
        Rémi Denis-Courmont <remi@...lab.net>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Aurelien Jarno <aurelien@...el32.net>,
        Mathieu Malaterre <malat@...ian.org>,
        Jan Newger <jannewger@...gle.com>
Subject: Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf
 counters

Hi Atish, Rémi,


On 19/07/2023 19:14, Atish Patra wrote:
> On Wed, Jul 19, 2023 at 7:46 AM Rémi Denis-Courmont <remi@...lab.net> wrote:
>> Le keskiviikkona 19. heinäkuuta 2023, 1.48.49 EEST Atish Patra a écrit :
>>>> Isn't RDTIM susceptible to interference from power management and CPU
>>>> frequency scaling? I suppose that RDCYCLE may behave differently depending
>>>> on PM in *some* designs, but that would still be way better than RDTIME
>>>> 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 that
>> is disabled at build time, the raw PMU cycle counter - which obviously leads
>> 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 in v5.


Thanks for all your comments and sorry for being slow here. I will 
improve the commit logs in the next version, that's an oversight, sorry 
about that.

Thanks again,

Alex


>
>>> My point was it should just do whatever it does for other ISA. RISC-V is 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).
>>
>> --
>> Rémi Denis-Courmont
>> http://www.remlab.net/
>>
>>
>>
>