From: "Rémi Denis-Courmont" <remi@remlab.net>
To: Atish Patra <atishp@atishpatra.org>
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>
Subject: Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters
Date: Tue, 18 Jul 2023 22:04:45 +0300 [thread overview]
Message-ID: <8756384.Zvl9fuB8X0@basile.remlab.net> (raw)
In-Reply-To: <CAOnJCUJreDYWZcSRr-pb1vwRg_LyU7TBG+b_9Ys8rVmM8_fvgA@mail.gmail.com>
Le tiistaina 18. heinäkuuta 2023, 21.45.15 EEST Atish Patra a écrit :
> > I agree that it's not only insecure but also incorrect. However it mostly
> > works. In fact I don't disagree with the change as such, but I think that
> > the commit messages are misleading and confusing. For a start, in one
> > place it says that it is not breaking user space and in another it says
> > basically the opposite.
>
> Agreed. We will improve the commit message to clarify that. That's also the
> reason I started this whole thread :)
>
> > (Unfortunately, not everybody agrees with the change. I can't seem to get
> > FFmpeg's checkasm tool fixed:
> > http://ffmpeg.org/pipermail/ffmpeg-devel/2023-July/312245.html )
>
> Why can't rdtime(equivalent of rdtsc) be used instead of rdcycle ?
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.
As far as benchmarking is concerned (_excluding_ system security), RDTIME
seems to have all the problems of RDCYCLE, and then some more, no?
> The perf syscall overhead is just one time setup thing during the
> start of the application.
> For counting the cycles before/after a loop, it still provides a
> direct CSR access in user mode.
I suppose that you allude to mmap() here? The (dumb) FFmpeg code is using the
ioctl() interface though, but that's just laziness.
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next prev parent reply other threads:[~2023-07-19 6:12 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-03 12:46 [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters Alexandre Ghiti
2023-07-03 12:46 ` [PATCH v4 01/10] perf: Fix wrong comment about default event_idx Alexandre Ghiti
2023-07-03 12:46 ` [PATCH v4 02/10] include: riscv: Fix wrong include guard in riscv_pmu.h Alexandre Ghiti
2023-07-03 12:46 ` [PATCH v4 03/10] riscv: Make legacy counter enum match the HW numbering Alexandre Ghiti
2023-07-14 8:01 ` Atish Patra
2023-07-03 12:46 ` [PATCH v4 04/10] drivers: perf: Rename riscv pmu sbi driver Alexandre Ghiti
2023-07-14 8:01 ` Atish Patra
2023-07-03 12:46 ` [PATCH v4 05/10] riscv: Prepare for user-space perf event mmap support Alexandre Ghiti
2023-07-14 8:03 ` Atish Patra
2023-07-03 12:46 ` [PATCH v4 06/10] drivers: perf: Implement perf event mmap support in the legacy backend Alexandre Ghiti
2023-07-14 8:03 ` Atish Patra
2023-07-03 12:46 ` [PATCH v4 07/10] drivers: perf: Implement perf event mmap support in the SBI backend Alexandre Ghiti
2023-07-14 8:46 ` Atish Patra
2023-07-20 8:36 ` Alexandre Ghiti
2023-07-03 12:46 ` [PATCH v4 08/10] Documentation: admin-guide: Add riscv sysctl_perf_user_access Alexandre Ghiti
2023-07-03 13:03 ` Andrew Jones
2023-07-04 12:28 ` Vince Weaver
2023-07-14 8:50 ` Atish Patra
2023-07-03 12:46 ` [PATCH v4 09/10] tools: lib: perf: Implement riscv mmap support Alexandre Ghiti
2023-07-14 9:29 ` Atish Patra
2023-07-03 12:46 ` [PATCH v4 10/10] perf: tests: Adapt mmap-basic.c for riscv Alexandre Ghiti
2023-07-14 9:29 ` Atish Patra
2023-07-14 9:07 ` [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters Atish Patra
2023-07-17 13:35 ` Rémi Denis-Courmont
2023-07-17 23:22 ` Atish Patra
2023-07-18 17:06 ` Rémi Denis-Courmont
2023-07-18 18:45 ` Atish Patra
2023-07-18 19:04 ` Rémi Denis-Courmont [this message]
2023-07-18 22:48 ` Atish Patra
2023-07-19 14:45 ` Rémi Denis-Courmont
2023-07-19 17:14 ` Atish Patra
2023-07-20 9:13 ` Alexandre Ghiti
2023-07-27 14:14 Alexandre Ghiti
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