Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs

[Nitesh Narayan Lal <nitesh@redhat.com>]

On 2/6/21 7:43 PM, Nitesh Narayan Lal wrote:
> On 2/5/21 5:23 PM, Thomas Gleixner wrote:
>> On Thu, Feb 04 2021 at 14:17, Nitesh Narayan Lal wrote:
>>> On 2/4/21 2:06 PM, Marcelo Tosatti wrote:
>>>>>> How about adding a new flag for isolcpus instead?
>>>>>>
>>>>> Do you mean a flag based on which we can switch the affinity mask to
>>>>> housekeeping for all the devices at the time of IRQ distribution?
>>>> Yes a new flag for isolcpus. HK_FLAG_IRQ_SPREAD or some better name.
>>> Does sounds like a nice idea to explore, lets see what Thomas thinks about it.

<snip>

>>> When the affinity mask of the interrupt at the time when it is actually
>>> requested contains an isolated CPU then nothing prevents the kernel from
>>> steering it at an isolated CPU. But that has absolutely nothing to do
>>> with that spreading thingy.
>>>
>>> The only difference which this change makes is the fact that the
>>> affinity hint changes. Nothing else.
>>>
> Thanks for the detailed explanation.
>
> Before I posted this patch, I was doing some debugging on a setup where I
> was observing some latency issues due to the iavf IRQs that were pinned on
> the isolated CPUs.
>
> Based on some initial traces I had this impression that the affinity hint
> or cpumask_local_spread was somehow playing a role in deciding the affinity
> mask of these IRQs. Although, that does look incorrect after going through
> your explanation.
> For some reason, with a kernel that had this patch when I tried creating
> VFs iavf IRQs always ended up on the HK CPUs.
>
> The reasoning for the above is still not very clear to me. I will investigate
> this further to properly understand this behavior.
>
>

After a little more digging, I found out why cpumask_local_spread change
affects the general/initial smp_affinity for certain device IRQs.

After the introduction of the commit:

    e2e64a932 genirq: Set initial affinity in irq_set_affinity_hint()

For all the drivers that set hint, initial affinity is set based on the
CPU retrieved from cpumask_local_spread. So in an environment where
irqbalance is disabled, these device IRQs remain on the CPUs that are
picked from cpumask_local_spread even though they are isolated. I think
the commit message of the reverted patch should have covered this as
well.

-- 
Thanks
Nitesh