linux-kernel - Re: [RFC PATCH v2 1/1] scsi: mpi3mr: Introduce smp_affinity

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Message-ID: <fcec4cd7-8e0a-4dca-a1b4-519b5e683c52@oracle.com>
Date: Fri, 20 Jun 2025 11:43:40 +0100
From: John Garry <john.g.garry@...cle.com>
To: Aaron Tomlin <atomlin@...mlin.com>
Cc: "Sean A." <sean@...e.io>,
        "James.Bottomley@...senpartnership.com"
 <James.Bottomley@...senpartnership.com>,
        "kashyap.desai@...adcom.com" <kashyap.desai@...adcom.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "mpi3mr-linuxdrv.pdl@...adcom.com" <mpi3mr-linuxdrv.pdl@...adcom.com>,
        "sreekanth.reddy@...adcom.com" <sreekanth.reddy@...adcom.com>,
        "sumit.saxena@...adcom.com" <sumit.saxena@...adcom.com>
Subject: Re: [RFC PATCH v2 1/1] scsi: mpi3mr: Introduce smp_affinity_enable
 module parameter

On 19/06/2025 20:35, Aaron Tomlin wrote:
> On Wed, Jun 18, 2025 at 07:49:16AM +0100, John Garry wrote:
>> BTW, if you use taskset to set the affinity of a process and ensure that
>> /sys/block/xxx/queue/rq_affinity is set so that we complete on same CPU as
>> submitted, then I thought that this would ensure that interrupts are not
>> bothering other CPUs.
> Hi John,
> 
> I'm trying to understand this better. If I'm not mistaken, modifying
> /sys/block/[device]/queue/rq_affinity impacts where requests are processed.
> Could you clarify how this would prevent an IRQ from being delivered to an
> isolated CPU?

If you echo 2 > /sys/block/[device]/queue/rq_affinity, then completions 
will occur on the same CPU which their originated. And through taskset, 
if you keep the processes generating traffic on a certain group of CPUs, 
then other CPUs should not see any loading from those same processes.

I am assuming that there is a one-to-one CPU <-> HW queue relationship.

That's my unverified idea...