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Linux Kernel Update Resolves Intel Hybrid CPU Frequency Scaling Issue with Disabled E-Cores

The fix for Intel hybrid CPU frequency scaling when disabling E-Cores has been implemented in the Linux kernel. This fix addresses an issue where the frequency scaling of hybrid CPUs was not functioning correctly when some of the E-Cores (Efficiency Cores) were disabled.

The Intel P-State CPU frequency scaling driver, used in the Linux kernel, recently underwent a fix to resolve an issue concerning inadequate CPU frequency scaling on hybrid processors. The fix was designed to provide better performance and improved power management capabilities for systems with such processors. It also ensures that these systems are able to hit peak frequencies while still maintaining power savings.

This is an important fix as hybrid processors are becoming increasingly popular in the market, which could have caused significant performance issues for many users. The Intel P-State driver now includes a new parameter, “intel_pstate_hybrid”, which can be used to enable or disable the feature when necessary.

This issue could result in inadequate CPU frequency scaling behavior when operating on a hybrid processor with disabled E cores. The Intel P-State driver is responsible for managing the frequency scaling of the CPU, ensuring optimal performance and power efficiency. However, a flaw was discovered that affected this driver’s behavior on hybrid processors with E cores disabled.

To rectify this issue, developers have implemented a fix within the Intel P-State CPU frequency scaling driver. This fix addresses the problematic CPU frequency scaling behavior observed on hybrid processors with disabled E cores. By applying this fix, users can expect improved performance and power efficiency when utilizing the Intel P-State driver on hybrid processors.

Intel Core processors from the recent Alder Lake and Raptor Lake generations are equipped with both performance cores (P-cores) and efficiency cores (E-cores). However, some users may opt to disable the E-cores for various reasons, such as to avoid any potential issues related to processor core scheduling.

Unfortunately, such users may have experienced incorrect behavior from Intel’s P-State driver prior to the Linux 6.5 kernel code merge this week.

The repair is targeted at correcting these issues, and it will likely be backported into previous versions of the Linux kernel as well. This should resolve any problems that users of these newer processors may have been experiencing when running the P-State driver on their systems.

When mapping the performance levels of the hardware P-States (HWP) to frequency values, the Intel P-State driver has been updated so that hybrid-capable systems with deactivated E-cores utilize the correct scaling factor. According to Intel Linux developer Srinivas Pandruvada:

“some system BIOS configurations may provide the option to disable E-cores, which is the root of the problem addressed by the fix. The CPUID feature for hybrids may no longer be configured with this update (Leaf 7 sub leaf 0, EDX[15] = 0). Like any other hybrid-enabled system, a scaling factor will determine HWP performance limitations.

The scaling factor application check will fail if the hybrid CPUID option is not enabled. Scaling should only be used if the nominal frequency and performance of the CPPC are verified. Suppose the nominal frequency is not a multiple of 100MHz of the notional performance, and the CPPC nominal frequency and nominal performance are established. In that case, the hybrid scaling factor must be used.

Since non-hybrid capable systems do not publish their nominal frequency field in CPPC, the test mentioned above fails for them, and this method may be used for all HWP systems without requiring an extra cpu model check.”

In a significant development for the Linux 6.5 kernel, the primary set of power management updates was successfully merged last week. This milestone marks a crucial step forward in enhancing the efficiency and performance of the Linux system.

The integration of these updates is expected to bring about notable improvements in power management capabilities, ensuring optimal power utilization and enhancing the overall user experience. 

With this successful merger, the Linux community eagerly anticipates the upcoming release of the Linux 6.5 kernel, which promises to deliver improved power management features.

Mehedi Hasan
Mehedi Hasan
Mehedi Hasan is a passionate enthusiast for technology. He admires all things tech and loves to help others understand the fundamentals of Linux, servers, networking, and computer security in an understandable way without overwhelming beginners. His articles are carefully crafted with this goal in mind - making complex topics more accessible.

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