Re: [PATCH v3] x86: intel_epb: Add earlyparam option to keep bias at performance

Dave Hansen
2023-12-05T15:26:26Z

On 12/5/23 05:23, Jack Allister wrote:
> There are certain scenarios where it may be intentional that the EPB was
> set at to 0/ENERGY_PERF_BIAS_PERFORMANCE on kernel boot. For example, in
> data centers a kexec/live-update of the kernel may be performed regularly.
>
> Usually this live-update is time critical and defaulting of the bias back
> to ENERGY_PERF_BIAS_NORMAL may actually be detrimental to the overall
> update time if processors' time to ramp up/boost are affected.

If this makes your kexecs 7 times faster, please say that here.

Could we also please make this less wishy-washy? "May actually be
detrimental" does not scream how critical this is for you.

> This patch introduces a kernel command line "intel_epb_no_override"
> which will leave the EPB at performance if during the restoration code path
> it is detected as such.

No "this patch", please:

https://www.kernel.org/doc/html/next/process/maintainer-tip.html

This also needs documentation of the parameter in
Documentation/admin-guide/kernel-parameters.txt.

Let me see if I can write a sane changelog, summarizing the discussion
here for posterity. If there's confusion about a v1 patch that's
cleared up in the discussion, it would be wonderful to capture that in
the v2 changelog as opposed to making minimal changes. How's this? I
think it captures some of the things that Rafael related and also
additional information about the use case that motivated this effort.

--

Buggy BIOSes set a sane boot-time Energy Performance Bias (EPB) that
causes overheating. The kernel overrides any boot-time EPB
"performance" bias to "normal" to avoid this.

<Hardware name here> platforms can tolerate a "performance" bias during
boot without overheating. In addition, because of <root cause(s) here>,
a kexec with a "normal" bias is seven times slower than "performance" to
perform the kexec. Boot time is critical when performing a
kexec/live-update of the kernel which is running guests VMs since boot
time appears as guest latency or downtime.

Introduce a command-line parameter, "intel_epb_no_override", to skip the
"performance"=>"normal" override. This allows folks to get a speedy
kexec without exposing other folks with wonky BIOSes to overheating.