Linux Lands Fix For Potentially Bogus Number Of Intel Hybrid CPU HT Siblings

Written by Michael Larabel in Intel on 28 May 2023 at 08:11 AM EDT. 2 Comments
A few days ago I wrote about a Linux kernel patch being prepared for fixing Intel hybrid CPU SMP/HT topology reporting due to the way the Linux kernel was currently counting the number of Hyper Threading siblings for each core. Fortunately, that fix which is apparently becomes more pressing for upcoming Meteor Lake processors, has now been picked up in time for today's Linux 6.4-rc4 release and is set for back-porting to stable kernel series.

Thomas Gleixner this morning sent in a single x86/urgent patch for 6.4-rc4. He summed up the issue as:
"Prevent a bogus setting for the number of HT siblings, which is caused by the CPUID evaluation trainwreck of X86. That recomputes the value for each CPU, so the last CPU "wins". That can cause completely bogus sibling values."

As noted in the prior Phoronix article, this could report a Meteor Lake laptop as potentially having 11 CPU sockets each with a single core when in reality it was a single socket laptop having 16 total cores. This topology information was conveyed to user-space for use by tools like lscpu.

Intel hybrid CPU

Linus Torvalds already went ahead and merged the straight-forward patch for this change that was surprisingly overlooked until recently.

The Linux 6.4-rc4 kernel is due out by end of day with another week's worth of various bug/regression fixes. Linux 6.4 stable should be out around the end of June or early July.
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