Re: [RFC][Patch v1 3/3] PCI: Limit pci_alloc_irq_vectors as per housekeeping CPUs

[Nitesh Narayan Lal]

On 9/10/20 3:22 PM, Marcelo Tosatti wrote:
> On Wed, Sep 09, 2020 at 11:08:18AM -0400, Nitesh Narayan Lal wrote:
>> This patch limits the pci_alloc_irq_vectors max vectors that is passed on
>> by the caller based on the available housekeeping CPUs by only using the
>> minimum of the two.
>>
>> A minimum of the max_vecs passed and available housekeeping CPUs is
>> derived to ensure that we don't create excess vectors which can be
>> problematic specifically in an RT environment. This is because for an RT
>> environment unwanted IRQs are moved to the housekeeping CPUs from
>> isolated CPUs to keep the latency overhead to a minimum. If the number of
>> housekeeping CPUs are significantly lower than that of the isolated CPUs
>> we can run into failures while moving these IRQs to housekeeping due to
>> per CPU vector limit.
>>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
>> ---
>>  include/linux/pci.h | 16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
>>
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index 835530605c0d..750ba927d963 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -38,6 +38,7 @@
>>  #include <linux/interrupt.h>
>>  #include <linux/io.h>
>>  #include <linux/resource_ext.h>
>> +#include <linux/sched/isolation.h>
>>  #include <uapi/linux/pci.h>
>>  
>>  #include <linux/pci_ids.h>
>> @@ -1797,6 +1798,21 @@ static inline int
>>  pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
>>  		      unsigned int max_vecs, unsigned int flags)
>>  {
>> +	unsigned int num_housekeeping = num_housekeeping_cpus();
>> +	unsigned int num_online = num_online_cpus();
>> +
>> +	/*
>> +	 * Try to be conservative and at max only ask for the same number of
>> +	 * vectors as there are housekeeping CPUs. However, skip any
>> +	 * modification to the of max vectors in two conditions:
>> +	 * 1. If the min_vecs requested are higher than that of the
>> +	 *    housekeeping CPUs as we don't want to prevent the initialization
>> +	 *    of a device.
>> +	 * 2. If there are no isolated CPUs as in this case the driver should
>> +	 *    already have taken online CPUs into consideration.
>> +	 */
>> +	if (min_vecs < num_housekeeping && num_housekeeping != num_online)
>> +		max_vecs = min_t(int, max_vecs, num_housekeeping);
>>  	return pci_alloc_irq_vectors_affinity(dev, min_vecs, max_vecs, flags,
>>  					      NULL);
>>  }
> If min_vecs > num_housekeeping, for example:
>
> /* PCI MSI/MSIx support */
> #define XGBE_MSI_BASE_COUNT     4
> #define XGBE_MSI_MIN_COUNT      (XGBE_MSI_BASE_COUNT + 1)
>
> Then the protection fails.

Right, I was ignoring that case.

>
> How about reducing max_vecs down to min_vecs, if min_vecs >
> num_housekeeping ?

Yes, I think this makes sense.
I will wait a bit to see if anyone else has any other comment and will post
the next version then.

>
-- 
Nitesh

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