Re: [RFC 2/3] kernel/cpu: Mark nonboot cpus as inactive when shutting down nonboot cpus

[Pingfan Liu <piliu@redhat.com>]

On Mon, Oct 27, 2025 at 06:06:32PM +0100, Thomas Gleixner wrote:
> On Wed, Oct 22 2025 at 20:13, Pingfan Liu wrote:
> > The previous patch lifted the deadline bandwidth check during the kexec
> 
> Once this is applied 'The previous patch' is meaningless.
> 

I will rephrase it.
> > process, which raises a potential issue: as the number of online CPUs
> > decreases, DL tasks may be crowded onto a few CPUs, which may starve the
> > CPU hotplug kthread. As a result, the hot-removal cannot proceed in
> > practice.  On the other hand, as CPUs are offlined one by one, all tasks
> > will eventually be migrated to the kexec CPU.
> >
> > Therefore, this patch marks all other CPUs as inactive to signal the
> 
> git grep "This patch" Documentation/process/
> 

I will rephrase it.
> > scheduler to migrate tasks to the kexec CPU during hot-removal.
> 
> I'm not seeing what this solves. It just changes the timing of moving
> tasks off to the boot CPU where they compete for the CPU for nothing.
> 
> When kexec() is in progress, then running user space tasks at all is a
> completely pointless exercise.
> 
> So the obvious solution to the problem is to freeze all user space tasks

I agree, but what about a less intrusive approach? Simply stopping the 
DL tasks should suffice, as everything works correctly without them.

I have a draft patch ready. Let's discuss it and go from there.

> when kexec() is invoked. No horrible hacks in the deadline scheduler and
> elsewhere required to make that work. No?
> 

To clarify, skipping the dl_bw_deactivate() validation is necessary 
because it prevents CPU hot-removal.


Thanks,

Pingfan