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Date:   Thu, 22 Sep 2022 22:10:21 +0200
From:   Andreas Mohr <andi@lisas.de>
To:     Andreas Mohr <andi@lisas.de>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        linux-kernel@vger.kernel.org, rafael@kernel.org, lenb@kernel.org,
        linux-acpi@vger.kernel.org, linux-pm@vger.kernel.org,
        dave.hansen@linux.intel.com, bp@alien8.de, tglx@linutronix.de,
        puwen@hygon.cn, mario.limonciello@amd.com, peterz@infradead.org,
        rui.zhang@intel.com, gpiccoli@igalia.com,
        daniel.lezcano@linaro.org, ananth.narayan@amd.com,
        gautham.shenoy@amd.com, Calvin Ong <calvin.ong@amd.com>,
        stable@vger.kernel.org, regressions@lists.linux.dev
Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
 based on the Zen microarchitecture
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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.