linux-kernel - Re: [PATCH v2] [tip: sched/core] sched: Move PLACE_LAG and RUN_TO

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Message-ID: <20250212091711.GA19118@noisy.programming.kicks-ass.net>
Date: Wed, 12 Feb 2025 10:17:11 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Cristian Prundeanu <cpru@...zon.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>,
	Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>,
	Ali Saidi <alisaidi@...zon.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Geoff Blake <blakgeof@...zon.com>, Csaba Csoma <csabac@...zon.com>,
	Bjoern Doebel <doebel@...zon.com>,
	Gautham Shenoy <gautham.shenoy@....com>,
	Joseph Salisbury <joseph.salisbury@...cle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-tip-commits@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2] [tip: sched/core] sched: Move PLACE_LAG and
 RUN_TO_PARITY to sysctl

On Tue, Feb 11, 2025 at 11:36:44PM -0600, Cristian Prundeanu wrote:
> Replacing CFS with the EEVDF scheduler in kernel 6.6 introduced
> significant performance degradation in multiple database-oriented
> workloads. This degradation manifests in all kernel versions using EEVDF,
> across multiple Linux distributions, hardware architectures (x86_64,
> aarm64, amd64), and CPU generations.
> 
> Testing combinations of available scheduler features showed that the
> largest improvement (short of disabling all EEVDF features) came from
> disabling both PLACE_LAG and RUN_TO_PARITY.
> 
> Moving PLACE_LAG and RUN_TO_PARITY to sysctl will allow users to override
> their default values and persist them with established mechanisms.

Nope -- you have knobs in debugfs, and that's where they'll stay. Esp.
PLACE_LAG is super dodgy and should not get elevated to anything
remotely official.

Also, FYI, by keeping these emails threaded in the old thread I nearly
missed them again. I'm not sure where this nonsense of keeping
everything in one thread came from, but it is bloody stupid.