linux-kernel - Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask in sched domain

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Date:   Mon, 14 Sep 2020 12:26:47 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     "Li\, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.li@...el.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask in sched domain


On 14/09/20 12:08, Qais Yousef wrote:
> On 09/14/20 11:31, Valentin Schneider wrote:
>>
>> On 12/09/20 00:04, Li, Aubrey wrote:
>> >>> +++ b/include/linux/sched/topology.h
>> >>> @@ -65,8 +65,21 @@ struct sched_domain_shared {
>> >>>     atomic_t	ref;
>> >>>     atomic_t	nr_busy_cpus;
>> >>>     int		has_idle_cores;
>> >>> +	/*
>> >>> +	 * Span of all idle CPUs in this domain.
>> >>> +	 *
>> >>> +	 * NOTE: this field is variable length. (Allocated dynamically
>> >>> +	 * by attaching extra space to the end of the structure,
>> >>> +	 * depending on how many CPUs the kernel has booted up with)
>> >>> +	 */
>> >>> +	unsigned long	idle_cpus_span[];
>> >>
>> >> Can't you use cpumask_var_t and zalloc_cpumask_var() instead?
>> >
>> > I can use the existing free code. Do we have a problem of this?
>> >
>>
>> Nah, flexible array members are the preferred approach here; this also
>
> Is this your opinion or a rule written somewhere I missed?

I don't think there's a written rule, but AIUI it is preferred by at
least Peter:

https://lore.kernel.org/linux-pm/20180612125930.GP12217@hirez.programming.kicks-ass.net/
https://lore.kernel.org/lkml/20180619110734.GO2458@hirez.programming.kicks-ass.net/

And my opinion is that, if you can, having fewer separate allocation is better.

>
>> means we don't let CONFIG_CPUMASK_OFFSTACK dictate where this gets
>> allocated.
>>
>> See struct numa_group, struct sched_group, struct sched_domain, struct
>> em_perf_domain...
>
> struct root_domain, struct cpupri_vec, struct generic_pm_domain,
> struct irq_common_data..
>
> Use cpumask_var_t.
>
> Both approach look correct to me, so no objection in principle. cpumask_var_t
> looks neater IMO and will be necessary once more than one cpumask are required
> in a struct.
>

You're right in that cpumask_var_t becomes necessary when you need more
than one mask. For those that use it despite requiring only one mask
(cpupri stuff, struct nohz too), I'm not sure.