linux-kernel - Re: [PATCH v3 03/10] sched/topology: Provide cfs_overload

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Date:   Fri, 9 Nov 2018 17:38:34 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Steve Sistare <steven.sistare@...cle.com>, mingo@...hat.com,
        peterz@...radead.org
Cc:     subhra.mazumdar@...cle.com, dhaval.giani@...cle.com,
        daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com,
        matt@...eblueprint.co.uk, umgwanakikbuti@...il.com,
        riel@...hat.com, jbacik@...com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, quentin.perret@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 03/10] sched/topology: Provide cfs_overload_cpus bitmap

Hi Steve,

On 09/11/2018 12:50, Steve Sistare wrote:
[...]
> @@ -482,6 +484,10 @@ static void update_top_cache_domain(int cpu)
>  	dirty_sched_domain_sysctl(cpu);
>  	destroy_sched_domains(tmp);
>  
> +	sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
> +	cfs_overload_cpus = (sd ? sd->shared->cfs_overload_cpus : NULL);
> +	rcu_assign_pointer(rq->cfs_overload_cpus, cfs_overload_cpus);
> +

Why not do this in update_top_cache_domain() where we also look for the
highest SD_SHARE_PKG_RESOURCES and setup shortcut pointers?

>  	update_top_cache_domain(cpu);
>  }
>  
> @@ -1619,9 +1625,19 @@ static void __sdt_free(const struct cpumask *cpu_map)
>  	}
>  }
>  
> +#define ZALLOC_MASK(maskp, nelems, node)				  \
> +	(!*(maskp) && !zalloc_sparsemask_node(maskp, nelems,		  \
> +					      SPARSEMASK_DENSITY_DEFAULT, \
> +					      GFP_KERNEL, node))	  \
> +
>  static int sd_llc_alloc(struct sched_domain *sd)
>  {
> -	/* Allocate sd->shared data here. Empty for now. */
> +	struct sched_domain_shared *sds = sd->shared;
> +	struct cpumask *span = sched_domain_span(sd);
> +	int nid = cpu_to_node(cpumask_first(span));
> +
> +	if (ZALLOC_MASK(&sds->cfs_overload_cpus, nr_cpu_ids, nid))

Mmm so this is called once on every CPU, but the !*(maskp) check in the
macro makes it so there is only one allocation per sd_llc_shared.

I wouldn't mind having that called out in a comment, or having the
pointer check done explicitly outside of the macro.

[...]