On 5/20/20 2:03 AM, Christoph Hellwig wrote: > On Wed, May 20, 2020 at 11:04:24AM +0800, Ming Lei wrote: >> On Wed, May 20, 2020 at 09:18:23AM +0800, Ming Lei wrote: >>> On Tue, May 19, 2020 at 05:30:00PM +0200, Christoph Hellwig wrote: >>>> On Tue, May 19, 2020 at 09:54:20AM +0800, Ming Lei wrote: >>>>> As Thomas clarified, workqueue hasn't such issue any more, and only other >>>>> per CPU kthreads can run until the CPU clears the online bit. >>>>> >>>>> So the question is if IO can be submitted from such kernel context? >>>> >>>> What other per-CPU kthreads even exist? >>> >>> I don't know, so expose to wider audiences. >> >> One user is io uring with IORING_SETUP_SQPOLL & IORING_SETUP_SQ_AFF, see >> io_sq_offload_start(), and it is a IO submission kthread. > > As far as I can tell that code is buggy, as it still needs to migrate > the thread away when the cpu is offlined. This isn't a per-cpu kthread > in the sene of having one for each CPU. > > Jens? It just uses kthread_create_on_cpu(), nothing home grown. Pretty sure they just break affinity if that CPU goes offline. -- Jens Axboe
