Re: [PATCH 11/14] irq: add support for allocating (and affinitizing) sets of IRQs

From: Jens Axboe
Date: Tue Oct 30 2018 - 12:42:12 EST

On 10/30/18 10:02 AM, Keith Busch wrote:
> On Tue, Oct 30, 2018 at 09:18:05AM -0600, Jens Axboe wrote:
>> On 10/30/18 9:08 AM, Keith Busch wrote:
>>> On Tue, Oct 30, 2018 at 08:53:37AM -0600, Jens Axboe wrote:
>>>> The sum of the set can't exceed the nvecs passed in, the nvecs passed in
>>>> should be the less than or equal to nvecs. Granted this isn't enforced,
>>>> and perhaps that should be the case.
>>> That should at least initially be true for a proper functioning
>>> driver. It's not enforced as you mentioned, but that's only related to
>>> the issue I'm referring to.
>>> The problem is pci_alloc_irq_vectors_affinity() takes a range, min_vecs
>>> and max_vecs, but a range of allowable vector allocations doesn't make
>>> sense when using sets.
>> I feel like we're going in circles here, not sure what you feel the
>> issue is now? The range is fine, whoever uses sets will need to adjust
>> their sets based on what pci_alloc_irq_vectors_affinity() returns,
>> if it didn't return the passed in desired max.
> Sorry, let me to try again.
> pci_alloc_irq_vectors_affinity() starts at the provided max_vecs. If
> that doesn't work, it will iterate down to min_vecs without returning to
> the caller. The caller doesn't have a chance to adjust its sets between
> iterations when you provide a range.
> The 'masks' overrun problem happens if the caller provides min_vecs
> as a smaller value than the sum of the set (plus any reserved).
> If it's up to the caller to ensure that doesn't happen, then min and
> max must both be the same value, and that value must also be the same as
> the set sum + reserved vectors. The range just becomes redundant since
> it is already bounded by the set.
> Using the nvme example, it would need something like this to prevent the
> 'masks' overrun:

OK, now I hear what you are saying. And you are right, the callers needs
to provide minvec == maxvec for sets, and then have a loop around that
to adjust as needed.

I'll make that change in nvme.

Jens Axboe