Re: [PATCH 2/2] nvme-pci: use blk_mq_max_nr_hw_queues() to calculate io queues

[Ming Lei]

On Mon, Jul 10, 2023 at 10:51:43AM -0600, Keith Busch wrote:
> On Mon, Jul 10, 2023 at 05:14:15PM +0800, Ming Lei wrote:
> > On Mon, Jul 10, 2023 at 08:41:09AM +0200, Christoph Hellwig wrote:
> > > On Sat, Jul 08, 2023 at 10:02:59AM +0800, Ming Lei wrote:
> > > > Take blk-mq's knowledge into account for calculating io queues.
> > > > 
> > > > Fix wrong queue mapping in case of kdump kernel.
> > > > 
> > > > On arm and ppc64, 'maxcpus=1' is passed to kdump command line, see
> > > > `Documentation/admin-guide/kdump/kdump.rst`, so num_possible_cpus()
> > > > still returns all CPUs.
> > > 
> > > That's simply broken.  Please fix the arch code to make sure
> > > it does not return a bogus num_possible_cpus value for these
> > 
> > That is documented in Documentation/admin-guide/kdump/kdump.rst.
> > 
> > On arm and ppc64, 'maxcpus=1' is passed for kdump kernel, and "maxcpu=1"
> > simply keep one of CPU cores as online, and others as offline.
> > 
> > So Cc our arch(arm & ppc64) & kdump guys wrt. passing 'maxcpus=1' for
> > kdump kernel.
> > 
> > > setups, otherwise you'll have to paper over it in all kind of
> > > drivers.
> > 
> > The issue is only triggered for drivers which use managed irq &
> > multiple hw queues.
> 
> Is the problem that the managed interrupt sets the effective irq
> affinity to an offline CPU? You mentioned observed timeouts; are you

Yes, the problem is that blk-mq only creates hctx0, so nvme-pci
translate it into hctx0's nvme_queue, this way is actually wrong, cause
blk-mq's view on queue topo isn't same with nvme's view.

> seeing the "completion polled" nvme message?

Yes, "completion polled" can be observed. Meantime the warning in
__irq_startup_managed() can be triggered from
nvme_timeout()->nvme_poll_irqdisable()->enable_irq().


Thanks,
Ming