Re: [PATCH v10 10/11] sched: add SD_PREFER_SIBLING for SMT level

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Vincent Guittot <vincent.guittot@linaro.org> [2015-02-27 16:54:13]:

> Add the SD_PREFER_SIBLING flag for SMT level in order to ensure that
> the scheduler will put at least 1 task per core.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> Reviewed-by: Preeti U. Murthy <preeti@linux.vnet.ibm.com>
> ---
>  kernel/sched/core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 29f7037..753f0a2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6240,6 +6240,7 @@ sd_init(struct sched_domain_topology_level *tl, int cpu)
>  	 */
>  
>  	if (sd->flags & SD_SHARE_CPUCAPACITY) {
> +		sd->flags |= SD_PREFER_SIBLING;
>  		sd->imbalance_pct = 110;
>  		sd->smt_gain = 1178; /* ~15% */
>  

Prefer siblings logic dates back to https://lkml.org/lkml/2009/8/27/210
and only used in update_sd_lb_stats() where we have
 
if (child && child->flags & SD_PREFER_SIBLING)
	 prefer_sibling = 1;

However what confuses me is why should we even look at a child domain's
flag to balance tasks across the current sched domain? Why cant we just
set and use a sd flag at current level than to look at child domain
flag?

> -- 
> 1.9.1
> 
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-- 
Thanks and Regards
Srikar Dronamraju