Linux 6.15.2 Fixes "Quite Dramatically...Potentially Dangerous" Idle Power Regression
Along with releasing Linux 6.14.11 today to end-of-life the Linux 6.14 kernel series, Greg Kroah-Hartman released Linux 6.15.2 as the newest stable point release. There is a notable fix here for the CPU idle power regressing on some systems since moving to Linux 6.15.
Linux 6.15.2 contains the latest fixes and other back-ports from Linux 6.16 Git. Making Linux 6.15.2 notable is picking up the back-ported revert for fixing a power regression affecting some systems with Linux 6.15.
As noted there, on some systems such as those booting with a "nosmt" kernel configuration it's possible that the CPU power use "rises quite dramatically". Linux power management subsystem maintainer and Intel engineer Rafael Wysocki explained with the revert commit:
This revert to avoid the problem is part of the Linux 6.15.2 changes.
In addition to Linux 6.15.2 and then Linux 6.14.11 EOL'ing that series, Linux 6.12.33 LTS was also released today for this kernel trio. The new kernels can be downloaded from kernel.org.
Linux 6.15.2 contains the latest fixes and other back-ports from Linux 6.16 Git. Making Linux 6.15.2 notable is picking up the back-ported revert for fixing a power regression affecting some systems with Linux 6.15.
As noted there, on some systems such as those booting with a "nosmt" kernel configuration it's possible that the CPU power use "rises quite dramatically". Linux power management subsystem maintainer and Intel engineer Rafael Wysocki explained with the revert commit:
"Namely, on such systems, SMT siblings permanently go offline early, when cpuidle has not been initialized yet, so after the above commit, hlt_play_dead() is called for them. Later on, when the processor attempts to enter a deep package C-state, including PC10 which is requisite for reaching minimum power in suspend-to-idle, it is not able to do that because of the SMT siblings staying in C1 (which they have been put into by HLT).
As a result, the idle power (including power in suspend-to-idle) rises quite dramatically on those systems with all of the possible consequences, which (needless to say) may not be expected by their users.
This issue is hard to debug and potentially dangerous, so it needs to be addressed as soon as possible in a way that will work for 6.15.y, hence the revert."
This revert to avoid the problem is part of the Linux 6.15.2 changes.
In addition to Linux 6.15.2 and then Linux 6.14.11 EOL'ing that series, Linux 6.12.33 LTS was also released today for this kernel trio. The new kernels can be downloaded from kernel.org.