Re: [RFC PATCH v5 4/7] sched: bias task wakeups to preferred semi-idle packages

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

* Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> [2008-12-11 23:13:04]:

> 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 |   17 +++++++++++++++++
>  1 files changed, 17 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 98345e4..939f2a1 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1027,6 +1027,23 @@ 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 &&

The p->mm check is racy, it needs to be done under task_lock(). The
best way to check for a kernel thread is get_task_mm(), followed by
put_task_mm() is the mm is not NULL. We also need to check to see if
the task is _hot_ on cpu. We should negate this optimization in case
chosen_wakeup_cpu is idle, so check for that as well.

> +		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.
> 
> 

-- 
	Balbir