Re: [PATCH v7 5/8] sched: bias task wakeups to preferred semi-idle packages

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

* Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2008-12-18 23:26:29]:

> Preferred wakeup cpu (from a semi idle package) has been
> nominated in find_busiest_group() in the previous patch.  Use
> this information in sched_mc_preferred_wakeup_cpu in function
> wake_idle() to bias task wakeups if the following conditions
> are satisfied:
>         - The present cpu that is trying to wakeup the process is
>           idle and waking the target process on this cpu will
>           potentially wakeup a completely idle package
>         - The previous cpu on which the target process ran is
>           also idle and hence selecting the previous cpu may
>           wakeup a semi idle cpu package
>         - The task being woken up is allowed to run in the
>           nominated cpu (cpu affinity and restrictions)
> 
> Basically if both the current cpu and the previous cpu on
> which the task ran is idle, select the nominated cpu from semi
> idle cpu package for running the new task that is waking up.
> 
> Cache hotness is considered since the actual biasing happens
> in wake_idle() only if the application is cache cold.
> 
> This technique will effectively move short running bursty jobs in
> a mostly idle system.
> 
> Wakeup biasing for power savings gets automatically disabled if
> system utilisation increases due to the fact that the probability
> of finding both this_cpu and prev_cpu idle decreases.
> 
> Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
> ---
> 
>  kernel/sched_fair.c |   18 ++++++++++++++++++
>  1 files changed, 18 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 98345e4..c82386c 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1027,6 +1027,24 @@ static int wake_idle(int cpu, struct task_struct *p)
>  	cpumask_t tmp;
>  	struct sched_domain *sd;
>  	int i;
> +	unsigned int chosen_wakeup_cpu;
> +	int this_cpu;
> +
> +	/*
> +	 * At POWERSAVINGS_BALANCE_WAKEUP level, if both this_cpu and prev_cpu
> +	 * are idle and this is not a kernel thread and this task's affinity
> +	 * allows it to be moved to preferred cpu, then just move!
> +	 */
> +
> +	this_cpu = smp_processor_id();
> +	chosen_wakeup_cpu =
> +		cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu;
> +
> +	if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP &&
> +		idle_cpu(cpu) && idle_cpu(this_cpu) &&
> +		p->mm && !(p->flags & PF_KTHREAD) &&
> +		cpu_isset(chosen_wakeup_cpu, p->cpus_allowed))
> +		return chosen_wakeup_cpu;
> 
>  	/*
>  	 * If it is idle, then it is the best cpu to run this task.

Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>

-- 
	Balbir