I'm not familiar with Linux kernel internals, or how scaling_cur_freq is implemented internally, but that doesn't look like a valid logic to me. If it takes an IPI (or two, as the commit message suggests) to read the core frequency, then make those IPIs. It doesn't matter how expensive it is - if the user wants to read the current frequency then he is willing to pay for that information. This likely won't be a frequent operation anyway. Providing an interface to read this information and then feeding bogus data through it is not acceptable, IMO.
-- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux-signed-lowlatency-hwe-6.5 in Ubuntu. https://bugs.launchpad.net/bugs/2051733 Title: Specifying nohz_full breaks CPU frequency reporting Status in linux-signed-lowlatency-hwe-6.5 package in Ubuntu: Confirmed Bug description: With the lowlatency kernel, if I specify "nohz_full=1-15" boot parameter then CPU frequency reporting doesn't work for the logical cores 1-15. That is, only logical core 0 shows varying CPU frequency in its /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq file, all other cores constantly show 800000 in their scaling_cur_freq files (which is the lowest supported frequency) regardless of the CPU load. Steps to reproduce: 1. Add "nohz_full=1-15" (specify the core numbers to include all logical cores except 0) to kernel boot options in /etc/default/grub. 2. Run `sudo update-grub` and reboot. 3. Upon booting, run a multithreaded workload. For example, run `openssl speed -multi $(nproc --all)`. 4. In another console, monitor CPU frequencies by running `watch cat /sys/devices/system/cpu/cpu[0-9]*/cpufreq/scaling_cur_freq`. Actual results: All cores specified in "nohz_full" parameter always report their lowest frequency. Despite that, the actual performance seems to be as if frequency scaling actually works (i.e. according to benchmarks, the performance seems to be similar with and without the "nohz_full" parameter). Expected results: All cores must report their actual frequency depending on the load. ProblemType: Bug DistroRelease: Ubuntu 22.04 Package: linux-image-6.5.0-15-lowlatency 6.5.0-15.15.1.1~22.04.1 ProcVersionSignature: Ubuntu 6.5.0-15.15.1.1~22.04.1-lowlatency 6.5.3 Uname: Linux 6.5.0-15-lowlatency x86_64 NonfreeKernelModules: nvidia_modeset nvidia ApportVersion: 2.20.11-0ubuntu82.5 Architecture: amd64 CasperMD5CheckResult: unknown CurrentDesktop: KDE Date: Tue Jan 30 23:39:51 2024 InstallationDate: Installed on 2015-05-01 (3196 days ago) InstallationMedia: Kubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422) SourcePackage: linux-signed-lowlatency-hwe-6.5 UpgradeStatus: Upgraded to jammy on 2022-05-14 (626 days ago) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux-signed-lowlatency-hwe-6.5/+bug/2051733/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
