Messages in this thread | | | From | Rémi Denis-Courmont <> | Subject | Re: [PATCH v4 00/10] riscv: Allow userspace to directly access perf counters | Date | Tue, 18 Jul 2023 22:04:45 +0300 |
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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.
-- レミ・デニ-クールモン http://www.remlab.net/
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