linux-kernel - Re: [PATCH] sched/fair: Skip cpus with no sched domain attached during NOHZ idle balance

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Date:   Fri, 22 Sep 2023 17:03:37 +0200
From:   Pierre Gondois <pierre.gondois@....com>
To:     "Zhang, Rui" <rui.zhang@...el.com>,
        "Lu, Aaron" <aaron.lu@...el.com>
Cc:     "tj@...nel.org" <tj@...nel.org>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "longman@...hat.com" <longman@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Pandruvada, Srinivas" <srinivas.pandruvada@...el.com>
Subject: Re: [PATCH] sched/fair: Skip cpus with no sched domain attached
 during NOHZ idle balance

Hi Rui,

On 9/20/23 09:24, Zhang, Rui wrote:
> Hi, Pierre,
> 
> Sorry for the late response. I'm still ramping up on the related code.

No worries, I also need to read the code,

> 
> On Thu, 2023-09-14 at 16:53 +0200, Pierre Gondois wrote:
>>
>>
>> On 9/14/23 11:23, Zhang, Rui wrote:
>>> Hi, Pierre,
>>>
>>>>
>>>> Yes right indeed,
>>>> This happens when putting a CPU offline (as you mentioned
>>>> earlier,
>>>> putting a CPU offline clears the CPU in the idle_cpus_mask).
>>>>
>>>> The load balancing related variables
>>>
>>> including?
>>
>> I meant the nohz idle variables in the load balancing, so I was
>> referring to:
>> (struct sched_domain_shared).nr_busy_cpus
>> (struct sched_domain).nohz_idle
>> nohz.idle_cpus_mask
>> nohz.nr_cpus
>> (struct rq).nohz_tick_stopped
> 
> IMO, the problem is that, for an isolated CPU,
> 1. it is not an idle cpu (nohz.idle_cpus_mask should be cleared)
> 2. it is not a busy cpu (sds->nr_busy_cpus should be decreased)
> 
> But current code does not have a third state to describe this, so we
> need to either
> 1. add extra logic, like on_null_domain() checks

I m not sure I understand, do you mean adding on_null_domain() in addition
to the one in the present patch ?

> or
> 2. rely on current logic, but update all related variables correctly,
> like you proposed.
> 
> But in any case, we should stick with one direction.
> 
> If we follow the first one, the original patch should be used, which
> IMO is simple and straight forward.
> If we follow the later one, we'd better audit and remove the current
> on_null_domain() usage at the same time. TBH, I'm not confident enough

Here aswell, I'm not sure I understand whether you are referring to
the on_null_domain() call in the present patch or to the on_null_domain()
calls in fair.c ?

Regards,
Pierre

> to make such a change. But if you want to propose something, I'd glad
> to test it.
> 
> thanks,
> rui
>