linux-kernel - Re: [PATCH] sched: cgroup SCHED

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Date:   Fri, 11 Jun 2021 18:43:04 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Paul Turner <pjt@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Steve Sistare <steven.sistare@...cle.com>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: cgroup SCHED_IDLE support

On 10/06/2021 21:14, Josh Don wrote:
> Hey Dietmar,
> 
> On Thu, Jun 10, 2021 at 5:53 AM Dietmar Eggemann
> <dietmar.eggemann@....com> wrote:
>>
>> Any reason why this should only work on cgroup-v2?
> 
> My (perhaps incorrect) assumption that new development should not
> extend v1. I'd actually prefer making this work on v1 as well; I'll
> add that support.
> 
>> struct cftype cpu_legacy_files[] vs. cpu_files[]
>>
>> [...]
>>
>>> @@ -11340,10 +11408,14 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
>>>
>>>  static DEFINE_MUTEX(shares_mutex);
>>>
>>> -int sched_group_set_shares(struct task_group *tg, unsigned long shares)
>>> +#define IDLE_WEIGHT sched_prio_to_weight[ARRAY_SIZE(sched_prio_to_weight) - 1]
>>
>> Why not 3 ? Like for tasks (WEIGHT_IDLEPRIO)?
>>
>> [...]
> 
> Went back and forth on this; on second look, I do think it makes sense
> to use the IDLEPRIO weight of 3 here. This gets converted to a 0,
> rather than a 1 for display of cpu.weight, which is also actually a
> nice property.

I'm struggling to see the benefit here.

For a taskgroup A: Why setting A/cpu.idle=1 to force a minimum A->shares
when you can set it directly via A/cpu.weight (to 1 (minimum))?

WEIGHT	   cpu.weight 	tg->shares

3	   0		3072

15	   1		15360

	   1		10240

`A/cpu.weight` follows cgroup-v2's `weights` `resource distribution
model`* but I can only see `A/cpu.idle` as a layer on top of it forcing
`A/cpu.weight` to get its minimum value?

*Documentation/admin-guide/cgroup-v2.rst