Re: [PATCH] sched/deadline: Derive root domain from active cpu in task's cpus_ptr

[Peter Zijlstra <peterz@infradead.org>]

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?