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Date:   Tue, 22 Sep 2020 09:54:58 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>, frederic@...nel.org,
        bhelgaas@...gle.com
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org, sassmann@...hat.com,
        jeffrey.t.kirsher@...el.com, jacob.e.keller@...el.com,
        jlelli@...hat.com, hch@...radead.org, mike.marciniszyn@...el.com,
        dennis.dalessandro@...el.com, thomas.lendacky@....com,
        jerinj@...vell.com, mathias.nyman@...el.com, jiri@...dia.com
Subject: Re: [RFC][Patch v1 3/3] PCI: Limit pci_alloc_irq_vectors as per
 housekeeping CPUs


On 9/10/20 3:31 PM, Nitesh Narayan Lal wrote:
> 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@...hat.com>
>>> ---
>>>  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.
>

Are there any other comments/concerns on this patch that I need to address in
the next posting?

-- 
Nitesh



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