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

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Message-ID: <20200911162853.xldy6fvvqph2lahj@e107158-lin.cambridge.arm.com>
Date:   Fri, 11 Sep 2020 17:28:54 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Aubrey Li <aubrey.li@...el.com>
Cc:     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,
        valentin.schneider@....com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask
 in sched domain

On 09/10/20 13:42, Aubrey Li wrote:
> Added idle cpumask to track idle cpus in sched domain. When a CPU
> enters idle, its corresponding bit in the idle cpumask will be set,
> and when the CPU exits idle, its bit will be cleared.
> 
> When a task wakes up to select an idle cpu, scanning idle cpumask
> has low cost than scanning all the cpus in last level cache domain,
> especially when the system is heavily loaded.
> 
> Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>
> ---
>  include/linux/sched/topology.h | 13 +++++++++++++
>  kernel/sched/fair.c            |  4 +++-
>  kernel/sched/topology.c        |  2 +-
>  3 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index fb11091129b3..43a641d26154 100644
> --- a/include/linux/sched/topology.h
> +++ 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?

The patch looks useful. Did it help you with any particular workload? It'd be
good to expand on that in the commit message.

Thanks

--
Qais Yousef