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

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Message-ID: <20250929135347.GH3289052@noisy.programming.kicks-ass.net>
Date: Mon, 29 Sep 2025 15:53:47 +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 Mon, Sep 29, 2025 at 09:36:02PM +0800, Pingfan Liu wrote:
> When testing kexec-reboot on a 144 cpus machine with
> isolcpus=managed_irq,domain,1-71,73-143 in kernel command line, I
> encounter the following bug:
> 
> [   97.114759] psci: CPU142 killed (polled 0 ms)
> [   97.333236] Failed to offline CPU143 - error=-16
> [   97.333246] ------------[ cut here ]------------
> [   97.342682] kernel BUG at kernel/cpu.c:1569!

> [   97.514379] Call trace:
> [   97.516874]  smp_shutdown_nonboot_cpus+0x104/0x128
> [   97.521769]  machine_shutdown+0x20/0x38
> [   97.525693]  kernel_kexec+0xc4/0xf0
> [   97.529260]  __do_sys_reboot+0x24c/0x278
> [   97.533272]  __arm64_sys_reboot+0x2c/0x40

> Tracking down this issue, I found that dl_bw_deactivate() returned
> -EBUSY, which caused sched_cpu_deactivate() to fail on the last CPU.
> When a CPU is inactive, its rd is set to def_root_domain. For an S-state

You mean a blocked task?

> deadline task (in this case, "cppc_fie"), it was not migrated to CPU0,
> and its task_rq() information is stale. As a result, its bandwidth is
> wrongly accounted into def_root_domain during domain rebuild.
> 
> This patch uses the rd from the run queue of still-active CPU to get the
> correct root domain.

That doesn't seem right in general. What if there are multiple root
domains; how does it know which to use?