Re: [PATCH-next 00/23] cgroup/cpuset: Enable runtime update of nohz_full and managed_irq CPUs

[Waiman Long <longman@redhat.com>]

On 7/1/26 10:22 AM, Frederic Weisbecker wrote:
> Le Thu, Jun 25, 2026 at 01:27:54AM -0400, Waiman Long a écrit :
>> On 6/24/26 2:34 AM, Jing Wu wrote:
>>>     3. Are there specific patches in your series where you would welcome
>>>        our contribution directly?
>> I have broken down the shutdown callback into separate portions as suggested
>> by Thomas. The other major change that I am working on is to try to shutdown
>> to only CPUHP_AP_OFFLINE state instead of all the way down to CPUHP_OFFLINE.
> What was the reason for that already? Can we perhaps ask the user to offline
> the target CPUs before toggling isolation on them?
The major problem about fully offlining the CPU is the CPU hotplug stop 
machine mechanism which put all the CPUs except the CPU to be offlined 
in a waiting loop within the IPI handler when the offline CPU is 
transitioning from CPUHP_TEARDOWN_CPU to  CPUHP_AP_IDLE_DEAD. If there 
is another active isolated partition running DPDK, for instance, it will 
break the low latency guarantee for a short duration.
>> That will require some adjustments to the nohz_full related hotplug
>> functions. I have some ideas of what needs to be done. However, I haven't
>> looked into RCU yet. I know RCU support changing the nocb mask for fully
>> offline CPUs, I will need to find out if it possible to do that for
>> partially offline CPUs.
> No because callbacks can still be enqueued at this stage. But we could
> manage to make it work with CPUHP_AP_IDLE_DEAD.

If we can only go as high as CPUHP_AP_IDLE_DEAD, we may as well go down 
all the way to CPUHP_OFFLINE as stop machine should be done at 
CPUHP_AP_IDLE_DEAD. In that case, we may have to break RCU out from 
HK_TYPE_KERNEL_NOISE and add a cpuset control switch for the system 
administrators to decide if they are willing to suffer a brief latency 
spike for an existing isolated partition or keep the RCU housekeeping 
mask unchanged to avoid that when creating a new or destroying an old 
isolated partition.

Cheers,
Longman

>
> Thanks.
>