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Subject: Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping
 CPUs
From:   Nitesh Narayan Lal <nitesh@redhat.com>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc:     Robin Murphy <robin.murphy@arm.com>, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, frederic@kernel.org,
        juri.lelli@redhat.com, abelits@marvell.com, bhelgaas@google.com,
        linux-pci@vger.kernel.org, rostedt@goodmis.org, mingo@kernel.org,
        peterz@infradead.org, davem@davemloft.net,
        akpm@linux-foundation.org, sfr@canb.auug.org.au,
        stephen@networkplumber.org, rppt@linux.vnet.ibm.com,
        jinyuqi@huawei.com, zhangshaokun@hisilicon.com
References: <20200625223443.2684-1-nitesh@redhat.com>
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Organization: Red Hat Inc,
Message-ID: <07c04bc7-27f0-9c07-9f9e-2d1a450714ef@redhat.com>
Date:   Thu, 4 Mar 2021 13:15:31 -0500
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On 2/11/21 10:55 AM, Nitesh Narayan Lal wrote:
> 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()
>

Continuing the conversation about the above commit and adding Jesse.
I was trying to understand the problem that the commit message explains
"The default behavior of the kernel is somewhat undesirable as all
requested interrupts end up on CPU0 after registration.", I have also been
trying to reproduce this behavior without the patch but I failed in doing
so, maybe because I am missing something here.

@Jesse Can you please explain? FWIU IRQ affinity should be decided based on
the default affinity mask.

The problem with the commit is that when we overwrite the affinity mask
based on the hinting mask we completely ignore the default SMP affinity
mask. If we do want to overwrite the affinity based on the hint mask we
should atleast consider the default SMP affinity.

-- 
Thanks
Nitesh