A Fix Is Out For The Intel Ice Lake Performance Drop On Linux With The Dell XPS 7390
The article earlier this week looked at how on recent versions of the Linux kernel the Intel Core i7 1065G7 performance was much lower than on older kernels... Running Linux 5.4 or even Linux 5.6 stable or Linux 5.7 Git meant around 20% lower performance compared to using the Linux 5.3 kernel, at least for this particular Dell XPS laptop that serves as my lone Ice Lake testbed. That article showing it to be a clear change as a result of a newer kernel came weeks (due to time constraints) after first showing the Dell Ice Lake performance dropping big time on Ubuntu 20.04 with its Linux 5.4 based kernel compared to Ubuntu 19.10.
With the Linux 5.4 kernel there is processor thermal device support for Ice Lake that exposes the PPCC power tables. It turns out that at least for the Dell 7390 2-in-1 that 9 Watts is specified by this power table exposed on Linux 5.4+ rather than the default 15 Watts. As a result, Intel's Thermal Daemon (thermald) is respecting that and limiting the CPU power consumption on recent kernels to just a 9W capacity rather than 15W. The exception is if using Intel's dptfxtract to properly read it -- but for more on that binary debacle, see Intel DPTF Adaptive Policy Being Reverse Engineered For Better Linux Ultrabook Support.
To avoid this situation though for those running Linux out-of-the-box on the 7390 ultrabook and any other affected systems, Intel has made changes to their Thermal Daemon.
Today they landed two changes to avoid polling power in non-PPCC cases and ignoring invalid PPCC maximum power limits. In the case of an invalid maximum power limit, it will be ignored rather than dropping to that lower value. Following that thermal work, Intel then released the 2.1 Thermal Daemon with these changes. Intel is now working on getting it backported for Ubuntu. It's making sure thermald is updated as opposed to a new Linux kernel release.
So at this point it's clear the Dell XPS 7390 is affected but it's not known what other Ice Lake systems could be affected with this change in Linux 5.4, but we'll see if any others surface with problematic power tables.
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