linux-kernel - Re: [PATCH 1/2] sched: Feature to decide if steal should update CPU capacity

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Message-ID: <20251028111813.GK3245006@noisy.programming.kicks-ass.net>
Date: Tue, 28 Oct 2025 12:18:13 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Srikar Dronamraju <srikar@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, Ben Segall <bsegall@...gle.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Nicholas Piggin <npiggin@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 1/2] sched: Feature to decide if steal should update CPU
 capacity

On Tue, Oct 28, 2025 at 04:12:54PM +0530, Srikar Dronamraju wrote:
> At present, scheduler scales CPU capacity for fair tasks based on time
> spent on irq and steal time. If a CPU sees irq or steal time, its
> capacity for fair tasks decreases causing tasks to migrate to other CPU
> that are not affected by irq and steal time. All of this is gated by
> NONTASK_CAPACITY.
> 
> In virtualized setups, a CPU that reports steal time (time taken by the
> hypervisor) can cause tasks to migrate unnecessarily to sibling CPUs that
> appear to be less busy, only for the situation to reverse shortly.
> 
> To mitigate this ping-pong behaviour, this change introduces a new
> scheduler feature flag: ACCT_STEAL which will control whether steal time
> contributes to non-task capacity adjustments (used for fair scheduling).

Please don't use sched_feat like this. If this is something that wants
to be set by architectures move it to a normal static_branch (like eg.
sched_energy_present, sched_asymc_cpucapacity, sched_cluster_active,
sched_smt_present, sched_numa_balancing etc.).