[SRU] [Jammy] [Kinetic] [PATCH 0/1] limit "Dummy wait" to old Intel

[Thadeu Lima de Souza Cascardo]

On Wed, Sep 28, 2022 at 10:58:00AM -0400, Jeff Lane wrote:
> Old, circa 2002 chipsets have a bug: they don't go idle when they are
> supposed to. So, a workaround was added to slow the CPU down and
> ensure that the CPU waits a bit for the chipset to actually go idle.
> This workaround is ancient and has been in place in some form since
> the original kernel ACPI implementation.
> 
> But, this workaround is very painful on modern systems. The "inl()"
> can take thousands of cycles (see Link: for some more detailed
> numbers and some fun kernel archaeology).
> 
> First and foremost, modern systems should not be using this code.
> Typical Intel systems have not used it in over a decade because it is
> horribly inferior to MWAIT-based idle.
> 
> Despite this, people do seem to be tripping over this workaround on
> AMD system today.
> 
> Limit the "dummy wait" workaround to Intel systems. Keep Modern AMD
> systems from tripping over the workaround. Remotely modern Intel
> systems use intel_idle instead of this code and will, in practice,
> remain unaffected by the dummy wait.
> 
> Small patch cleanly picks to both Kinetic and Jammy.
> 

Has this been tested on AMD systems and shown to improve the situation? Which
AMD systems?

Does this affect the OEM-5.17 kernel as well?

Cascardo.

> 
> Dave Hansen (1):
>   ACPI: processor idle: Practically limit "Dummy wait" workaround to old
>     Intel systems
> 
>  drivers/acpi/processor_idle.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> -- 
> 2.34.1
> 
> 
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