Re: [PATCH v1 0/2] x86/smp: Fix power regression introduced by commit 96040f7273e2

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 28, 2025 at 02:53:13PM +0200, Rafael J. Wysocki wrote:
> Hi Everyone,
> 
> Commit 96040f7273e2 ("x86/smp: Eliminate mwait_play_dead_cpuid_hint()")
> that shipped in 6.15 introduced a nasty power regression on systems that
> start with "nosmt" in the kernel command line which prevents it from entering
> deep package idle states (for instance, PC10) later on.  Idle power, including
> power in suspend-to-idle, goes up significantly on those systems as a result.
> 
> Address this by reverting commit 96040f7273e2 (patch [1/2]) and using a
> different approach, which is to retain mwait_play_dead_cpuid_hint() and
> still prefer it to hlt_play_dead() in case it is needed when cpuidle is
> not available, but prefer cpuidle_play_dead() to it by default (patch [2/2]).

I don't understand. The revert says the reason it regresses is that it
goes into play_dead before cpuidle is initialized. The fix is then to
call cpuidle first.

But if cpuidle isn't initialized yet, how does that fix anything?