linux-kernel - Re: [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and RUN_TO

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Message-ID: <20241101130531.GW9767@noisy.programming.kicks-ass.net>
Date: Fri, 1 Nov 2024 14:05:31 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Cristian Prundeanu <cpru@...zon.com>
Cc: "Gautham R. Shenoy" <gautham.shenoy@....com>,
	linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	Bjoern Doebel <doebel@...zon.com>,
	Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>,
	Geoff Blake <blakgeof@...zon.com>, Ali Saidi <alisaidi@...zon.com>,
	Csaba Csoma <csabac@...zon.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and
 RUN_TO_PARITY and move them to sysctl

On Mon, Oct 28, 2024 at 11:57:49PM -0500, Cristian Prundeanu wrote:

> My testing with SCHED_BATCH is meanwhile concluded. It did reduce the 
> regression to less than half - but only with WAKEUP_PREEMPTION enabled. 
> When using NO_WAKEUP_PREEMPTION, there was no performance change compared 
> to SCHED_OTHER.

Because BATCH affects wakeup-preemption, and if there isn't any ever, it
makes no difference.

A BATCH task will not preempt another BATCH task, the only thing driving
preemption is slice exhaustion -- and we can now set slice per task to
match with the 'work' cycle.

> (At the risk of stating the obvious, using SCHED_BATCH only to get back to 
> the default CFS performance is still only a workaround,

It is not really -- it is impossible to schedule all the various
workloads without them telling us what they really like. The quest is to
find interfaces that make sense and are implementable. But fundamentally
tasks will have to start telling us what they need. We've long since ran
out of crystal balls.