linux-kernel - Re: [RFC PATCH] sched&net: avoid over-pulling tasks due to network interrupts

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Date:   Mon, 8 Nov 2021 08:27:50 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Barry Song <21cnbao@...il.com>
Cc:     David Miller <davem@...emloft.net>, kuba@...nel.org,
        Eric Dumazet <edumazet@...gle.com>, pabeni@...hat.com,
        fw@...len.de, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>,
        Guodong Xu <guodong.xu@...aro.org>,
        yangyicong <yangyicong@...wei.com>, shenyang39@...wei.com,
        tangchengchang@...wei.com, Barry Song <song.bao.hua@...ilicon.com>,
        Libo Chen <libo.chen@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFC PATCH] sched&net: avoid over-pulling tasks due to network
 interrupts



On 11/8/21 1:27 AM, Peter Zijlstra wrote:
> On Mon, Nov 08, 2021 at 07:08:09AM +1300, Barry Song wrote:
>> On Sat, Nov 6, 2021 at 1:25 AM Peter Zijlstra <peterz@...radead.org> wrote:
>>>
>>> On Fri, Nov 05, 2021 at 06:51:36PM +0800, Barry Song wrote:
>>>> From: Barry Song <song.bao.hua@...ilicon.com>
>>>>
>>>> In LPC2021, both Libo Chen and Tim Chen have reported the overpull
>>>> of network interrupts[1]. For example, while running one database,
>>>> ethernet is located in numa0, numa1 might be almost idle due to
>>>> interrupts are pulling tasks to numa0 because of wake_up affine.
>>>> I have seen the same problem. One way to solve this problem is
>>>> moving to a normal wakeup in network rather than using a sync
>>>> wakeup which will be more aggressively pulling tasks in scheduler
>>>> core.
>>>>
>>>> On kunpeng920 with 4numa, ethernet is located at numa0, storage
>>>> disk is located at numa2. While using sysbench to connect this
>>>> mysql machine, I am seeing numa1 is idle though numa0,2 and 3
>>>> are quite busy.
>>>>
>>>
>>>> I am not saying this patch is exactly the right approach, But I'd
>>>> like to use this RFC to connect the people of net and scheduler,
>>>> and start the discussion in this wider range.
>>>
>>> Well the normal way would be to use multi-queue crud and/or receive
>>> packet steering to get the interrupt/wakeup back to the cpu that data
>>> came from.
>>
>> The test case has been a multi-queue ethernet and irqs are balanced
>> to NUMA0 by irqbalanced or pinned to NUMA0 where the card is located
>> by the script like:
>> #!/bin/bash
>> irq_list=(`cat /proc/interrupts | grep network_name| awk -F: '{print $1}'`)
>> cpunum=0
>> for irq in ${irq_list[@]}
>> do
>> echo $cpunum > /proc/irq/$irq/smp_affinity_list
>> echo `cat /proc/irq/$irq/smp_affinity_list`
>> (( cpunum+=1 ))
>> done
>>
>> I have heard some people are working around this issue  by pinning
>> multi-queue IRQs to multiple NUMAs which can spread interrupts and
>> avoid over-pulling tasks to one NUMA only, but lose ethernet locality?
> 
> So you're doing explicitly the wrong thing with your script above and
> then complain the scheduler follows that and destroys your data
> locality?
> 
> The network folks made RPS/RFS specifically to spread the processing of
> the packets back to the CPUs/Nodes the TX happened on to increase data
> locality. Why not use that?
> 

+1

This documentation should describe how this can be done

Documentation/networking/scaling.rst

Hopefully it is not completely outdated.