Date: Tue, 23 Jul 2013 17:47:36 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Rakib Mullick <rakib.mullick@gmail.com>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH, RFC] sched: update_top_cache_domain only at the times of
 building sched domain.
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On Tue, Jul 23, 2013 at 09:44:17PM +0600, Rakib Mullick wrote:
>  Currently, update_top_cache_domain() is called whenever schedule domain is built or destroyed. But, the following
> callpath shows that they're at the same callpath and can be avoided update_top_cache_domain() while destroying schedule
> domain and update only at the times of building schedule domains.
> 
> 	partition_sched_domains()
> 		detach_destroy_domain()
> 			cpu_attach_domain()
> 				update_top_cache_domain()
> 		build_sched_domains()
> 			cpu_attach_domain()
> 				update_top_cache_domain()
> 		

Does it really matter?

> 
> Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
> ---
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b7c32cb..8c6fee4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5102,8 +5102,8 @@ static void update_top_cache_domain(int cpu)
>   * Attach the domain 'sd' to 'cpu' as its base domain. Callers must
>   * hold the hotplug lock.
>   */
> -static void
> -cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
> +static void cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd,
> +				int cpu, bool update_cache_domain)
>  {
>  	struct rq *rq = cpu_rq(cpu);
>  	struct sched_domain *tmp;
> @@ -5138,7 +5138,8 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
>  	rcu_assign_pointer(rq->sd, sd);
>  	destroy_sched_domains(tmp, cpu);
>  
> -	update_top_cache_domain(cpu);
> +	if (update_cache_domain)
> +		update_top_cache_domain(cpu);
>  }
>  
>  /* cpus with isolated domains */
> @@ -6021,7 +6022,7 @@ static int build_sched_domains(const struct cpumask *cpu_map,
>  	rcu_read_lock();
>  	for_each_cpu(i, cpu_map) {
>  		sd = *per_cpu_ptr(d.sd, i);
> -		cpu_attach_domain(sd, d.rd, i);
> +		cpu_attach_domain(sd, d.rd, i, 1);
>  	}
>  	rcu_read_unlock();
>  
> @@ -6109,7 +6110,7 @@ static void detach_destroy_domains(const struct cpumask *cpu_map)
>  
>  	rcu_read_lock();
>  	for_each_cpu(i, cpu_map)
> -		cpu_attach_domain(NULL, &def_root_domain, i);
> +		cpu_attach_domain(NULL, &def_root_domain, i, 0);
>  	rcu_read_unlock();
>  }

This just makes the code uglier for no gain afaict.

If you really need to do this, key off @sd == NULL or something.
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