Re: PCI, isolcpus, and irq affinity

["Sean V Kelley" <sean.v.kelley@intel.com>]

On 12 Oct 2020, at 9:58, Bjorn Helgaas wrote:

> [+cc Christoph, Thomas, Nitesh]
>
> On Mon, Oct 12, 2020 at 09:49:37AM -0600, Chris Friesen wrote:
>> I've got a linux system running the RT kernel with threaded irqs.  
>> On
>> startup we affine the various irq threads to the housekeeping CPUs, 
>> but I
>> recently hit a scenario where after some days of uptime we ended up 
>> with a
>> number of NVME irq threads affined to application cores instead (not 
>> good
>> when we're trying to run low-latency applications).
>
> pci_alloc_irq_vectors_affinity() basically just passes affinity
> information through to kernel/irq/affinity.c, and the PCI core doesn't
> change affinity after that.
>
>> Looking at the code, it appears that the NVME driver can in some 
>> scenarios
>> end up calling pci_alloc_irq_vectors_affinity() after initial system
>> startup, which seems to determine CPU affinity without any regard for 
>> things
>> like "isolcpus" or "cset shield".
>>
>> There seem to be other reports of similar issues:
>>
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1831566
>>
>> It looks like some SCSI drivers and virtio_pci_common.c will also 
>> call
>> pci_alloc_irq_vectors_affinity(), though I'm not sure if they would 
>> ever do
>> it after system startup.
>>
>> How does it make sense for the PCI subsystem to affine interrupts to 
>> CPUs
>> which have explicitly been designated as "isolated"?
>
> This recent thread may be useful:
>
>   https://lore.kernel.org/linux-pci/20200928183529.471328-1-nitesh@redhat.com/
>
> It contains a patch to "Limit pci_alloc_irq_vectors() to housekeeping
> CPUs".  I'm not sure that patch summary is 100% accurate because IIUC
> that particular patch only reduces the *number* of vectors allocated
> and does not actually *limit* them to housekeeping CPUs.
>
> Bjorn


Chris,

Are you attempting a tick-less run?  I’ve seen the NO_HZ_FULL (full 
dynticks) feature behave somewhat inconsistently when PREEMPT_RT is 
enabled.  The timer ticks suppression feature can at times appear to be 
not functioning. I’m curious about how you are attempting to isolate 
the cores.

Thanks,

Sean