Re: [PATCH v2 3/3] lib/group_cpus.c: honor housekeeping config when grouping CPUs

Hannes Reinecke
2024-07-01T08:43:22Z

On 7/1/24 09:21, Ming Lei wrote:
> On Mon, Jul 01, 2024 at 09:08:32AM +0200, Daniel Wagner wrote:
>> On Sun, Jun 30, 2024 at 09:39:59PM GMT, Ming Lei wrote:
>>>> Make group_cpus_evenly aware of isolcpus configuration and use the
>>>> housekeeping CPU mask as base for distributing the available CPUs into
>>>> groups.
>>>>
>>>> Fixes: 11ea68f553e2 ("genirq, sched/isolation: Isolate from handling managed interrupts")
>>>
>>> isolated CPUs are actually handled when figuring out irq effective mask,
>>> so not sure how commit 11ea68f553e2 is wrong, and what is fixed in this
>>> patch from user viewpoint?
>>
>> IO queues are allocated/spread on the isolated CPUs and if there is an
>> thread submitting IOs from an isolated CPU it will cause noise on the
>> isolated CPUs. The question is this a use case you need/want to support?
>
> I have talked RH Openshift team weeks ago and they have such usage.
>
> userspace is free to run any application from isolated CPUs via 'taskset
> -c' even though 'isolcpus=' is passed from command line.
>
> Kernel can not add such new constraint on userspace.
>
>> We have customers who are complaining that even with isolcpus provided
>> they still see IO noise on the isolated CPUs.
>
> That is another issue, which has been fixed by the following patch:
>
> a46c27026da1 blk-mq: don't schedule block kworker on isolated CPUs
>
Hmm. Just when I thought I understood the issue ...

How is this supposed to work, then, given that I/O can be initiated
from the isolated CPUs?
I would have accepted that we have two scheduling domains, blk-mq is
spread across all cpus, and the blk-mq cpusets are arranged according
to the isolcpu settings.
Then we can initiate I/O from the isolated cpus, and the scheduler
would 'magically' ensure that everything is only run on isolated cpus.

But that patch would completely counteract such a setup, as during
I/O we more often than not will invoke kblockd, which then would cause
cross-talk on non-isolated cpus.

What is the idea here?

Confused,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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