Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors based on the Zen microarchitecture

[Andreas Mohr]

On Thu, Sep 22, 2022 at 09:42:15PM +0200, Andreas Mohr wrote:
> So one can see where my profiling effort went
> (*optimizing* things, not degrading them)
> --> hints that current Zen3-originating effort is not
> about a regression in the "regression bug" sense -
> merely a (albeit rather appreciable/sizeable... congrats!)
> performance deterioration vs.
> an optimal (currently non-achieved) software implementation state
> (also: of PORT-based handling [vs. MWAIT], mind you!).

I'd like to add a word of caution here:

AFAIK power management (here: ACPI Cx) handling generally is
about a painful *tradeoff* between
achieving best-possible performance (that's
the respectable Zen3 32MB/s vs. 33MB/s argument) and
achieving maximum power savings.
We all know that one can configure the system for
non-idle mode (idle=poll cmdline?) and
achieve record numbers in performance (...*and* power consumption - ouch!).

Current decision/implementation aspects AFAICS:
- why is the Zen3 config used here choosing
  less-favourable(?) PORT-based operation mode?
- Zen3 is said to not have the STPCLK# issue
  (- but then what about other more modern chipsets?)

--> we need to achieve (hopefully sufficiently precisely) a solution which
takes into account Zen3 STPCLK# improvements while
preserving "accepted" behaviour/requirements on *all* STPCLK#-hampered chipsets
("STPCLK# I/O wait is default/traditional handling"?).

Greetings

Andreas Mohr

-- 
GNU/Linux. It's not the software that's free, it's you.