Re: [RFC PATCH v5 3/7] sched: nominate preferred wakeup cpu

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2008-12-11 23:12:57]:

> When the system utilisation is low and more cpus are idle,
> then the process waking up from sleep should prefer to
> wakeup an idle cpu from semi-idle cpu package (multi core
> package) rather than a completely idle cpu package which
> would waste power.
> 
> Use the sched_mc balance logic in find_busiest_group() to
> nominate a preferred wakeup cpu.
> 
> This info can be sored in appropriate sched_domain, but
> updating this info in all copies of sched_domain is not
> practical.  Hence this information is stored in root_domain
> struct which is one copy per partitioned sched domain.
> The root_domain can be accessed from each cpu's runqueue
> and there is one copy per partitioned sched domain.
> 
> Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
> ---
> 
>  kernel/sched.c |   12 ++++++++++++
>  1 files changed, 12 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 6bea99b..0918677 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -493,6 +493,14 @@ struct root_domain {
>  #ifdef CONFIG_SMP
>  	struct cpupri cpupri;
>  #endif
> +#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
> +	/*
> +	 * Preferred wake up cpu nominated by sched_mc balance that will be
> +	 * used when most cpus are idle in the system indicating overall very
> +	 * low system utilisation. Triggered at POWERSAVINGS_BALANCE_WAKEUP(2)

Is the root domain good enough?

What is POWERSAVINGS_BALANCE_WAKEUP(2), is it sched_mc == 2?

> +	 */
> +	unsigned int sched_mc_preferred_wakeup_cpu;
> +#endif
>  };
> 
>  /*
> @@ -3407,6 +3415,10 @@ out_balanced:
> 
>  	if (this == group_leader && group_leader != group_min) {
>  		*imbalance = min_load_per_task;
> +		if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP) {

OK, it is :) (for the question above). Where do we utilize the set
sched_mc_preferred_wakeup_cpu?

> +			cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu =
> +					first_cpu(group_leader->cpumask);

Everytime we balance, we keep replacing rd->sched_mc_preferred_wake_up
with group_lead->cpumask? My big concern is that we do this without
checking if the group_leader has sufficient capacity (after it will
pull in tasks since we made the checks for nr_running and capacity).

> +		}
>  		return group_min;
>  	}
>  #endif
> 
> 

-- 
	Balbir