Re: [PATCH 1/8] blk-mq: add blk_mq_max_nr_hw_queues()

Re: [PATCH 1/8] blk-mq: add blk_mq_max_nr_hw_queues()

[Ming Lei]

On Wed, Jul 12, 2023 at 03:00:17PM +0200, Christoph Hellwig wrote:
> On Wed, Jul 12, 2023 at 08:54:48PM +0800, Ming Lei wrote:
> > +/* Max nr_hw_queues for each hw queue type */
> > +unsigned int blk_mq_max_nr_hw_queues(void)
> > +{
> > +	if (is_kdump_kernel())
> > +		return 1;
> > +	return nr_cpu_ids;
> 
> Again, these is_kdump_kernel hacks don't make any sense.   The amount
> of maximum available CPU needs to come through a proper API, and we
> need to use it, not add hacks like this.
> 
> The only thing that makes sense here is to find the last CPU
> in cpu_possible_mask, and for kdump kernels to ensure that number
> is 1 or whatever low value they want.

It doesn't matter how many cpus are available, given at least one
cpu is online.

The problem is that blk_mq_alloc_tag_set() forces to set nr_hw_queues
as 1 for kdump kernel, that is why blk_mq_max_nr_hw_queues() has to
return 1 for kdump kernel.

We have to tell driver that blk-mq only supports 1 queue for kdump
kernel.

Or:

Thomas, can we disable managed irq for kdump kernel and switch to
non-managed irq? Then we can avoid driver's change. I'd suggest
this way if it is possible.

Thanks,
Ming

Re: [PATCH 1/8] blk-mq: add blk_mq_max_nr_hw_queues()

[Christoph Hellwig]

On Wed, Jul 12, 2023 at 09:16:11PM +0800, Ming Lei wrote:
> The problem is that blk_mq_alloc_tag_set() forces to set nr_hw_queues
> as 1 for kdump kernel, that is why blk_mq_max_nr_hw_queues() has to
> return 1 for kdump kernel.

Well, let's fix that first and work from there.  Same argument against
that deep magic applies there as well.

> Thomas, can we disable managed irq for kdump kernel and switch to
> non-managed irq? Then we can avoid driver's change. I'd suggest
> this way if it is possible.

Why the heck would we?

Re: [PATCH 1/8] blk-mq: add blk_mq_max_nr_hw_queues()

[Ming Lei]

On Wed, Jul 12, 2023 at 03:19:25PM +0200, Christoph Hellwig wrote:
> On Wed, Jul 12, 2023 at 09:16:11PM +0800, Ming Lei wrote:
> > The problem is that blk_mq_alloc_tag_set() forces to set nr_hw_queues
> > as 1 for kdump kernel, that is why blk_mq_max_nr_hw_queues() has to
> > return 1 for kdump kernel.
> 
> Well, let's fix that first and work from there.  Same argument against
> that deep magic applies there as well.

In short, driver needs to figure out nr_hw_queues first by hardware info,
then pass it to blk_mq_alloc_tag_set(), but blk_mq_alloc_tag_set() changes it,
so inconsistency is caused.

The only solution in this way is to tell driver the max supported
number from the beginning, that is what this patchset is doing.

> 
> > Thomas, can we disable managed irq for kdump kernel and switch to
> > non-managed irq? Then we can avoid driver's change. I'd suggest
> > this way if it is possible.
> 
> Why the heck would we?

IMO irq kernel doesn't make sense in kdump kernel, which is very
resource limited and has to be reliable.

PCI_IRQ_AFFINITY can be just one hint, pci_alloc_irq_vectors_affinity()
still allocates affinity in managed way, then queue mapping can work
just fine, and the only difference is that genirq handles this irqs
as non-manged wrt. migration.

This way should solve queue mapping issue, but driver still allocates
lots of queues, which take resource useless. So looks we still have to
fix drivers.


Thanks, 
Ming