linux-kernel - Re: [EXT] Re: [Patch v4 1/3] lib: Restrict cpumask_local

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Message-ID: <a5132281-53e8-c75e-8085-7a44fa805625@marvell.com>
Date:   Fri, 29 Jan 2021 09:34:42 -0800
From:   Alex Belits <abelits@...vell.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Robin Murphy <robin.murphy@....com>,
        <linux-kernel@...r.kernel.org>, <linux-api@...r.kernel.org>,
        <frederic@...nel.org>, <juri.lelli@...hat.com>,
        <bhelgaas@...gle.com>, <linux-pci@...r.kernel.org>,
        <rostedt@...dmis.org>, <mingo@...nel.org>, <peterz@...radead.org>,
        <davem@...emloft.net>, <akpm@...ux-foundation.org>,
        <sfr@...b.auug.org.au>, <stephen@...workplumber.org>,
        <rppt@...ux.vnet.ibm.com>, <jinyuqi@...wei.com>,
        <zhangshaokun@...ilicon.com>
Subject: Re: [EXT] Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to
 houskeeping CPUs

On 1/29/21 06:23, Marcelo Tosatti wrote:
> External Email
> 
> ----------------------------------------------------------------------
> On Fri, Jan 29, 2021 at 08:55:20AM -0500, Nitesh Narayan Lal wrote:
>>
>> On 1/28/21 3:01 PM, Thomas Gleixner wrote:
>>> On Thu, Jan 28 2021 at 13:59, Marcelo Tosatti wrote:
>>>>> The whole pile wants to be reverted. It's simply broken in several ways.
>>>> I was asking for your comments on interaction with CPU hotplug :-)
>>> Which I answered in an seperate mail :)
>>>
>>>> So housekeeping_cpumask has multiple meanings. In this case:
>>> ...
>>>
>>>> So as long as the meaning of the flags are respected, seems
>>>> alright.
>>> Yes. Stuff like the managed interrupts preference for housekeeping CPUs
>>> when a affinity mask spawns housekeeping and isolated is perfectly
>>> fine. It's well thought out and has no limitations.
>>>
>>>> Nitesh, is there anything preventing this from being fixed
>>>> in userspace ? (as Thomas suggested previously).
>>> Everything with is not managed can be steered by user space.
>>>
>>> Thanks,
>>>
>>>          tglx
>>>
>>
>> So, I think the conclusion here would be to revert the change made in
>> cpumask_local_spread via the patch:
>>   - lib: Restrict cpumask_local_spread to housekeeping CPUs
>>
>> Also, a similar case can be made for the rps patch that went in with
>> this:
>>  - net: Restrict receive packets queuing to housekeeping CPUs
> 
> Yes, this is the userspace solution:
> 
> https://lkml.org/lkml/2021/1/22/815
> 
> Should have a kernel document with this info and examples
> (the network queue configuration as well). Will
> send something.
> 
>>  + net: accept an empty mask in /sys/class/net/*/queues/rx-*/rps_cpus
>>
>> I am not sure about the PCI patch as I don't think we can control that from
>> the userspace or maybe I am wrong?
> 
> You mean "lib: Restrict cpumask_local_spread to housekeeping CPUs" ?
> 

If we want to do it from userspace, we should have something that 
triggers it in userspace. Should we use udev for this purpose?

-- 
Alex