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 12:08:10 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Valentin Schneider <valentin.schneider@....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 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?

> 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.

> 
> >>
> >> The patch looks useful. Did it help you with any particular workload? It'd be
> >> good to expand on that in the commit message.
> >>
> > Odd, that included in patch v1 0/1, did you receive it?

Aubrey,

Sorry I didn't see that no. It's important justification to be part of the
commit message, I think worth adding it.

Thanks

--
Qais Yousef