linux-kernel - Re: [PATCH] sched/fair: Introduce priority load balance for CFS

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Date:   Sat, 12 Nov 2022 10:51:11 +0800
From:   Song Zhang <zhangsong34@...wei.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
CC:     <mingo@...hat.com>, <peterz@...radead.org>,
        <juri.lelli@...hat.com>, <mcgrof@...nel.org>,
        <keescook@...omium.org>, <yzaikin@...gle.com>,
        <dietmar.eggemann@....com>, <rostedt@...dmis.org>,
        <bsegall@...gle.com>, <mgorman@...e.de>, <bristot@...hat.com>,
        <vschneid@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: Introduce priority load balance for CFS

Hi, Vincent

On 2022/11/3 17:22, Vincent Guittot wrote:
> On Thu, 3 Nov 2022 at 10:20, Song Zhang <zhangsong34@...wei.com> wrote:
>>
>>
>>
>> On 2022/11/3 16:33, Vincent Guittot wrote:
>>> On Thu, 3 Nov 2022 at 04:01, Song Zhang <zhangsong34@...wei.com> wrote:
>>>>
>>>> Thanks for your reply!
>>>>
>>>> On 2022/11/3 2:01, Vincent Guittot wrote:
>>>>> On Wed, 2 Nov 2022 at 04:54, Song Zhang <zhangsong34@...wei.com> wrote:
>>>>>>
>>>>>
>>>>> This really looks like a v3 of
>>>>> https://lore.kernel.org/all/20220810015636.3865248-1-zhangsong34@huawei.com/
>>>>>
>>>>> Please keep versioning.
>>>>>
>>>>>> Add a new sysctl interface:
>>>>>> /proc/sys/kernel/sched_prio_load_balance_enabled
>>>>>
>>>>> We don't want to add more sysctl knobs for the scheduler, we even
>>>>> removed some. Knob usually means that you want to fix your use case
>>>>> but the solution doesn't make sense for all cases.
>>>>>
>>>>
>>>> OK, I will remove this knobs later.
>>>>
>>>>>>
>>>>>> 0: default behavior
>>>>>> 1: enable priority load balance for CFS
>>>>>>
>>>>>> For co-location with idle and non-idle tasks, when CFS do load balance,
>>>>>> it is reasonable to prefer migrating non-idle tasks and migrating idle
>>>>>> tasks lastly. This will reduce the interference by SCHED_IDLE tasks
>>>>>> as much as possible.
>>>>>
>>>>> I don't agree that it's always the best choice to migrate a non-idle task 1st.
>>>>>
>>>>> CPU0 has 1 non idle task and CPU1 has 1 non idle task and hundreds of
>>>>> idle task and there is an imbalance between the 2 CPUS: migrating the
>>>>> non idle task from CPU1 to CPU0 is not the best choice
>>>>>
>>>>
>>>> If the non idle task on CPU1 is running or cache hot, it cannot be
>>>> migrated and idle tasks can also be migrated from CPU1 to CPU0. So I
>>>> think it does not matter.
>>>
>>> What I mean is that migrating non idle tasks first is not a universal
>>> win and not always what we want.
>>>
>>
>> But migrating online tasks first is mostly a trade-off that
>> non-idle(Latency Sensitive) tasks can obtain more CPU time and minimize
>> the interference caused by IDLE tasks. I think this makes sense in most
>> cases, or you can point out what else I need to think about it ?
>>
>> Best regards.
>>
>>>>
>>>>>>
>>>>>> Testcase:
>>>>>> - Spawn large number of idle(SCHED_IDLE) tasks occupy CPUs
>>>>>
>>>>> What do you mean by a large number ?
>>>>>
>>>>>> - Let non-idle tasks compete with idle tasks for CPU time.
>>>>>>
>>>>>> Using schbench to test non-idle tasks latency:
>>>>>> $ ./schbench -m 1 -t 10 -r 30 -R 200
>>>>>
>>>>> How many CPUs do you have ?
>>>>>
>>>>
>>>> OK, some details may not be mentioned.
>>>> My virtual machine has 8 CPUs running with a schbench process and 5000
>>>> idle tasks. The idle task is a while dead loop process below:
>>>
>>> How can you care about latency when you start 10 workers on 8 vCPUs
>>> with 5000 non idle threads ?
>>>
>>
>> No no no... spawn 5000 idle(SCHED_IDLE) processes not 5000 non-idle
>> threads, and with 10 non-idle schbench workers on 8 vCPUs.
> 
> yes spawn 5000 idle tasks but my point remains the same
> 

I am so sorry that I have not received your reply for a long time, and I 
am still waiting for it anxiously. In fact, migrating non-idle tasks 1st 
works well in most scenarios, so it maybe possible to add a 
sched_feat(LB_PRIO) to enable or disable that. Finally, I really hope 
you can give me some better advice.

Best regards.

Song Zhang