Re: [PATCH v2] sched: update_top_cache_domain only at the times of building sched domain.

[Michael Wang <wangyun@linux.vnet.ibm.com>]

Hi, Rakib

On 07/24/2013 01:42 AM, 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()

IMHO, cpu_attach_domain() and update_top_cache_domain() should be
paired, below patch will open a window which 'rq->sd == NULL' while
'sd_llc != NULL', isn't it?

I don't think we have the promise that before we rebuild the stuff
correctly, no one will utilize 'sd_llc'...

Further more, what will happen if the old sd was freed after next rcu
work cycle while 'sd_llc' still hold the reference for some victims?

Thus I do suggest we leave the things untouched since the benefit we get
is too less, not worth the risk...

Regards,
Michael Wang

> 		build_sched_domains()
> 			cpu_attach_domain()
> 				update_top_cache_domain()
> 
> Changes since v1: use sd to determine when to skip, courtesy PeterZ
> 
> Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
> ---
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b7c32cb..387fb66 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -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 (sd)
> +		update_top_cache_domain(cpu);
>  }
> 
>  /* cpus with isolated domains */
> 
> 
> 
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