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New Alder Lake Linux bug is similar to Ryzen CPPC bug on Windows 11, patch incoming

Intel Core text on a CPU mock-up

When Intel introduced its Alder Lake architecture back at its 2021 Architecture Day event, the company touted its performance optimization surrounding Microsoft's Windows 11. However, the same can not be said for Linux it seems. As only a few days ago, we reported on an Alder Lake bug that was causing performance loss on the upcoming Linux 5.16 kernel; and yesterday, a new patch was submitted for another issue detected on Alder Lake.

The new issue revolves around Collaborative Processor Performance Control (CPPC) or fastest core prioritization and is very reminiscent of the recent Ryzen CPPC2 bug on Windows 11.

Alder Lake has a new Performance Hybrid architecture consisting of both "Big" cores (E-core) and "Bigger" cores (P-core), but currently, Linux is unable to identify these separately when the system is in an overclocked state. As a consequence, the OS assigns equal performance potential to all the cores irrespective of whether they are P-cores or E-cores.

This leads to a conflict with Intel's Turbo Boost Max Technology (ITMT) which helps to schedule priority single-threaded workloads on the fastest cores. However, since all cores now have equal performance values assigned, ITMT fails to work in such overclocked systems.

The upcoming patch hopes to fix this problem with the help of MSR_HWP_CAPABILITIES or Hardware-Controlled P-States (HWP) whenever available. But the patch would probably not work on older legacy systems lacking MSR_HWP_CAPABILITIES.

via Phoronix

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