linux-kernel - Re: [PATCH] sched/core: Push tasks on force idle

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Message-ID: <20251128133822.GB4067720@noisy.programming.kicks-ass.net>
Date: Fri, 28 Nov 2025 14:38:22 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Fernand Sieber <sieberf@...zon.com>
Cc: mingo@...hat.com, vincent.guittot@...aro.org, abusse@...zon.de,
	dwmw@...zon.co.uk, gmazz@...zon.de, jschoenh@...zon.de,
	kprateek.nayak@....com, linux-kernel@...r.kernel.org,
	liuyuxua@...zon.com, rkagan@...zon.de, vineethr@...ux.ibm.com,
	nh-open-source@...zon.com
Subject: Re: [PATCH] sched/core: Push tasks on force idle

On Fri, Nov 28, 2025 at 03:19:54PM +0200, Fernand Sieber wrote:
> When a cpu enters force idle, it will
> 1) try to steal cookie matching tasks from other CPUs
> 2) do the newidle balance
> 
> If the stealing fails, we are out of options to get out of force idle
> properly. New idle balance might decide to pull other tasks, but they won't
> necessarily be matching anyways.
> 
> Introduce a step in between where we try to push the runnable tasks that
> are blocked in force idle to a more suitable CPU.
> 
> === Testing setup ===
> 
> Similar setup as in:
> https://lore.kernel.org/lkml/20251127202719.963766-1-sieberf@amazon.com
> 
> Testing is aimed at measuring perceived guest noise on hypervisor system
> with time shared scenarios.
> 
> Setup is on system where the load is nearing 100% which should allow no
> steal time. The system has 64 CPUs, with 8 VMs, each VM using core
> scheduling with 8 vCPUs per VM, time shared.
> 
> 7 VMs are running stressors (`stress-ng --cpu 0`) while the last VM is
> running the hwlat tracer with a width of 100ms, a period of 300ms, and
> a threshold of 100us. Each VM runs a cookied non vCPU VMM process that
> adds a light level of noise which forces some level of load balancing.
> 
> The test scenario is ran 10x60s and the average noise is measured (we use
> breaches scaled up to period/width to estimate noise).
> 
> === Testing results ===
> 
> Baseline noise: 1.20%
> After patch noise: 0.66% (-45%)

This is similar to that other patch, what happens if you combine the
two?