linux-kernel - Re: [PATCH] sched/deadline: Derive root domain from active cpu in task's cpus

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Message-ID: <20250930090334.GI4067720@noisy.programming.kicks-ass.net>
Date: Tue, 30 Sep 2025 11:03:34 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Pingfan Liu <piliu@...hat.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] sched/deadline: Derive root domain from active cpu in
 task's cpus_ptr

On Tue, Sep 30, 2025 at 09:47:33AM +0800, Pingfan Liu wrote:

> > > This patch uses the rd from the run queue of still-active CPU to get the
> > > correct root domain.
> >
> 
> Sorry that I haven't explained it clearly. I mean the still-active CPU
> in task->cpus_ptr,
> 
> > That doesn't seem right in general. What if there are multiple root
> > domains; how does it know which to use?
> >
> 
> In the case of task->cpus_ptr, there should be only one root domain, right?

IIRC there was a corner case somewhere; something like clearing the old
cpuset load_balance flag on the root domain would not iterate all tasks
or so.

The result would be tasks with all-set cpumasks (the default value)
spread over multiple root domains. Every task would be caught in
whatever root domain it was at the time of toggle.

This might have been fixed, but I can't remember.