Linux 4.14 Ensures The "Core Performance Boost" Bit Gets Set For AMD Ryzen CPUs
Written by Michael Larabel in AMD on 17 October 2017 at 03:55 PM EDT. 17 Comments
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Recently making waves in our forums was talk of a kernel patch to address a case where the AMD CPB (Core Performance Boost) isn't being exposed by Ryzen processors. Here's more details on that and some benchmarks.

Being talked about recently is f7f3dc0: "CPUID Fn8000_0007_EDX[CPB] is wrongly 0 on models up to B1. But they do support CPB (AMD's Core Performance Boosting cpufreq CPU feature), so fix that."

Basically the bit for indicating Core Performance Boost support indicates it's disabled for Ryzen CPUs while it should be enabled for hitting the boost frequencies on these processors. This patch isn't for Linux 4.15 but was already merged as a fix for Linux 4.14.

I confirmed the change with a Ryzen 7 1800X that is stepping 1 and indicated from /proc/cpuinfo via the flags line you can look for the presence of "cpb." (For reference, EPYC is stepping 2 and thus unaffected by this error.) Booting to Linux 4.13, the CPB string wasn't shown but is in fact reported when booting to the Linux 4.14 kernel.

Via the Ryzen 7 1800X setup I then proceeded to run some benchmarks to see if the performance was any difference with the new kernel:





But with a range of tests run, there isn't any real performance change to find out of the variety of tests executed.

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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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