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

[Jacob Keller <jacob.e.keller@intel.com>]

On 10/26/2020 3:13 PM, Jakub Kicinski wrote:
> On Mon, 26 Oct 2020 22:50:45 +0100 Thomas Gleixner wrote:
>> On Mon, Oct 26 2020 at 14:11, Jacob Keller wrote:
>>> On 10/26/2020 1:11 PM, Thomas Gleixner wrote:  
>>>> On Mon, Oct 26 2020 at 12:21, Jacob Keller wrote:  
>>>>> Are there drivers which use more than one interrupt per queue? I know
>>>>> drivers have multiple management interrupts.. and I guess some drivers
>>>>> do combined 1 interrupt per pair of Tx/Rx..  It's also plausible to to
>>>>> have multiple queues for one interrupt .. I'm not sure how a single
>>>>> queue with multiple interrupts would work though.  
>>>>
>>>> For block there is always one interrupt per queue. Some Network drivers
>>>> seem to have seperate RX and TX interrupts per queue.  
>>> That's true when thinking of Tx and Rx as a single queue. Another way to
>>> think about it is "one rx queue" and "one tx queue" each with their own
>>> interrupt...
>>>
>>> Even if there are devices which force there to be exactly queue pairs,
>>> you could still think of them as separate entities?  
>>
>> Interesting thought.
>>
>> But as Jakub explained networking queues are fundamentally different
>> from block queues on the RX side. For block the request issued on queue
>> X will raise the complete interrupt on queue X.
>>
>> For networking the TX side will raise the TX interrupt on the queue on
>> which the packet was queued obviously or should I say hopefully. :)
>>
>> But incoming packets will be directed to some receive queue based on a
>> hash or whatever crystallball logic the firmware decided to implement.
>>
>> Which makes this not really suitable for the managed interrupt and
>> spreading approach which is used by block-mq. Hrm...
>>
>> But I still think that for curing that isolation stuff we want at least
>> some information from the driver. Alternative solution would be to grant
>> the allocation of interrupts and queues and have some sysfs knob to shut
>> down queues at runtime. If that shutdown results in releasing the queue
>> interrupt (via free_irq()) then the vector exhaustion problem goes away.
>>
>> Needs more thought and information (for network oblivious folks like
>> me).
> 
> One piece of information that may be useful is that even tho the RX
> packets may be spread semi-randomly the user space can still control
> which queues are included in the mechanism. There is an indirection
> table in the HW which allows to weigh queues differently, or exclude
> selected queues from the spreading. Other mechanisms exist to filter
> flows onto specific queues.
> 
> IOW just because a core has an queue/interrupt does not mean that
> interrupt will ever fire, provided its excluded from RSS.
> 
> Another piece is that by default we suggest drivers allocate 8 RX
> queues, and online_cpus TX queues. The number of queues can be
> independently controlled via ethtool -L. Drivers which can't support
> separate queues will default to online_cpus queue pairs, and let
> ethtool -L only set the "combined" parameter.
> 

I know the Intel drivers usually have defaulted to trying to maintain
queue pairs. I do not believe this is technically a HW restriction, but
it is heavily built into the way the drivers work today.

> There are drivers which always allocate online_cpus interrupts, 
> and then some of them will go unused if #qs < #cpus.
> 
> 

Right.

> My unpopular opinion is that for networking devices all the heuristics
> we may come up with are going to be a dead end. We need an explicit API
> to allow users placing queues on cores, and use managed IRQs for data
> queues. (I'm assuming that managed IRQs will let us reliably map a MSI-X
> vector to a core :))
> 

I don't think it is that unpopular... This is the direction I'd like to
see us go as well.