On 5/20/20 1:41 PM, Thomas Gleixner wrote:

Jens Axboe [off-list ref] writes:

quoted

On 5/20/20 8:45 AM, Jens Axboe wrote:

quoted

It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure
they just break affinity if that CPU goes offline.

Just checked, and it works fine for me. If I create an SQPOLL ring with
SQ_AFF set and bound to CPU 3, if CPU 3 goes offline, then the kthread
just appears unbound but runs just fine. When CPU 3 comes online again,
the mask appears correct.

When exactly during the unplug operation is it unbound?

When the CPU has been fully offlined. I check the affinity mask, it
reports 0. But it's still being scheduled, and it's processing work.
Here's an example, PID 420 is the thread in question:

[root@archlinux cpu3]# taskset -p 420
pid 420's current affinity mask: 8
[root@archlinux cpu3]# echo 0 > online 
[root@archlinux cpu3]# taskset -p 420
pid 420's current affinity mask: 0
[root@archlinux cpu3]# echo 1 > online 
[root@archlinux cpu3]# taskset -p 420
pid 420's current affinity mask: 8

So as far as I can tell, it's working fine for me with the goals
I have for that kthread.

-- 
Jens Axboe