Re: [RFC][PATCH 4/8] sched: add smt_gain

[Gautham R Shenoy <ego@in.ibm.com>]

On Tue, Sep 01, 2009 at 10:34:35AM +0200, Peter Zijlstra wrote:
> The idea is that multi-threading a core yields more work capacity than
> a single thread, provide a way to express a static gain for threads.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  include/linux/sched.h    |    1 +
>  include/linux/topology.h |    1 +
>  kernel/sched.c           |    8 +++++++-
>  3 files changed, 9 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -930,6 +930,7 @@ struct sched_domain {
>  	unsigned int newidle_idx;
>  	unsigned int wake_idx;
>  	unsigned int forkexec_idx;
> +	unsigned int smt_gain;
>  	int flags;			/* See SD_* */
>  	enum sched_domain_level level;
> 
> Index: linux-2.6/include/linux/topology.h
> ===================================================================
> --- linux-2.6.orig/include/linux/topology.h
> +++ linux-2.6/include/linux/topology.h
> @@ -99,6 +99,7 @@ int arch_update_cpu_topology(void);
>  				| SD_SHARE_CPUPOWER,	\
>  	.last_balance		= jiffies,		\
>  	.balance_interval	= 1,			\
> +	.smt_gain		= 1178,	/* 15% */	\

/* 15% of SCHED_LOAD_SCALE */ , I suppose.

>  }
>  #endif
>  #endif /* CONFIG_SCHED_SMT */
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -8490,9 +8490,15 @@ static void init_sched_groups_power(int 
>  		weight = cpumask_weight(sched_domain_span(sd));
>  		/*
>  		 * SMT siblings share the power of a single core.
> +		 * Usually multiple threads get a better yield out of
> +		 * that one core than a single thread would have,
> +		 * reflect that in sd->smt_gain.
>  		 */
> -		if ((sd->flags & SD_SHARE_CPUPOWER) && weight > 1)
> +		if ((sd->flags & SD_SHARE_CPUPOWER) && weight > 1) {
> +			power *= sd->smt_gain;
>  			power /= weight;
> +			power >>= SCHED_LOAD_SHIFT;
> +		}
>  		sg_inc_cpu_power(sd->groups, power);
>  		return;
>  	}
> 
> -- 

-- 
Thanks and Regards
gautham