Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs

[Nitesh Narayan Lal]

On 10/23/20 4:58 AM, Peter Zijlstra wrote:
> On Thu, Oct 22, 2020 at 01:47:14PM -0400, Nitesh Narayan Lal wrote:
>
>> Hi Peter,
>>
>> So based on the suggestions from you and Thomas, I think something like the
>> following should do the job within pci_alloc_irq_vectors_affinity():
>>
>> +       if (!pci_is_managed(dev) && (hk_cpus < num_online_cpus()))
>> +               max_vecs = clamp(hk_cpus, min_vecs, max_vecs);
>>
>> I do know that you didn't like the usage of "hk_cpus < num_online_cpus()"
>> and to an extent I agree that it does degrade the code clarity.
> It's not just code clarity; I simply don't understand it. It feels like
> a band-aid that breaks thing.
>
> At the very least it needs a ginormous (and coherent) comment that
> explains:
>
>  - the interface
>  - the usage
>  - this hack

That make sense.

>
>> However, since there is a certain inconsistency in the number of vectors
>> that drivers request through this API IMHO we will need this, otherwise
>> we could cause an impact on the drivers even in setups that doesn't
>> have any isolated CPUs.
> So shouldn't we then fix the drivers / interface first, to get rid of
> this inconsistency?
>

Considering we agree that excess vector is a problem that needs to be
solved across all the drivers and that you are comfortable with the other
three patches in the set. If I may suggest the following:

- We can pick those three patches for now, as that will atleast fix a
  driver that is currently impacting RT workloads. Is that a fair
  expectation?

- In the meanwhile, I will start looking into individual drivers that
  consume this API to find out if there is a co-relation that can be
  derived between the max_vecs and number of CPUs. If that exists then I
  can go ahead and tweak the API's max_vecs accordingly. However, if this
  is absolutely random then I can come up with a sane comment
  before this check that covers the list of items you suggested.

- I also want to explore the comments made by Thomas which may take
  some time.


-- 
Thanks
Nitesh

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