Re: [PATCH] sched: Nominate the idle load balancer from a semi-idle group.

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2008-09-23 at 15:49 +0530, Gautham R Shenoy wrote:
> sched: Nominate the idle load balancer from a semi-idle group.
> 
> From: Gautham R Shenoy <ego@in.ibm.com>
> 
> Currently  the first cpu in the nohz cpu mask is nominated as
> the idle load balancer.
> 
> However, this could be a cpu from an idle group,
> thereby not yiedling the expected power savings.
> 
> Improve the logic to pick an idle cpu from a semi-idle group
> for performing the task of idle load balancing, when
> sched_mc/smt_powersavings is enabled.
> 
> This patch has no effect when sched_mc/smt_powersavings is turned off.
> 
> The patch is based on linux-2.6.27-rc5 and depends on the fix for
> sched_mc_powersavings posted here--> http://lkml.org/lkml/2008/9/5/135

Is already in sched-devel...

> SPEC-Power Results: on a large multi-socket multi-core x86.
> ----------------------------------------------------------------
> Summary with the patch:
>          9% improvement in the overall score
>          8% decrease in the Power consumption at 100% utilization.

Seems to me that that's a contradiction, if it were utilized at 100%
we'd never hit the idle path and all this code would just sit there not
doing anything much.

> 	12% decrease in the Power consumption at idle.
> 
> ----------------------------------------------------------------
> kernel_version |Final Score |ssj ops | 100% Watts | Idle Watts |
> ---------------|------------|--------|-------------------------|
> 2.6.27-rc5     |   x        |   y    |    z       |    w       |
> ---------------|------------|--------|------------|------------|
> 2.6.27-rc5 +   |  1.05x     | 0.98y  |  0.94z     |  0.92w     |
> sched_mc_fix   |            |        |            |            |
> ---------------|------------|--------|------------|------------|
> 2.6.27-rc5 +   |  1.09x     | 0.97y  |  0.92z     |  0.88w     |
> sched_mc_fix + |            |        |            |            |
> this_patch     |            |        |            |            |
> ----------------------------------------------------------------
> 
> Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
> ---
> 
>  kernel/sched.c |   86 ++++++++++++++++++++++++++++++++++++++++++++++++++------
>  1 files changed, 77 insertions(+), 9 deletions(-)
> 
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index cc1f81b..c186719 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -3960,6 +3960,82 @@ static void run_rebalance_domains(struct softirq_action *h)
>  #endif
>  }
>  
> +#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
> +/**
> + * lowest_powersavings_sd: Returns the lowest level of power savings
> + * domain of the given cpu.
> + * @cpu: The cpu whose lowest level of power savings domain to
> + * be returned.
> + *
> + * Power savings domain is the sched_domain where in we perform
> + * load balancing for power savings. Such a domain would have
> + * SD_POWERSAVINGS_BALANCE bit set in it's flags.
> + */
> +static inline struct sched_domain *lowest_powersavings_sd(int cpu)
> +{
> +	struct sched_domain *sd;
> +
> +	for_each_domain(cpu, sd)
> +		if (sd && (sd->flags & SD_POWERSAVINGS_BALANCE))
> +			break;
> +
> +	return sd;
> +}
> +
> +/**
> + * for_each_powersavings_domain: Iterates over all the power savings
> + * scheduler domains (see comment for lowest_powersavings_sd) from
> + * the lowest to the highest.
> + * @cpu: The cpu whose powersavings_sd we're iterating over.
> + * @sd: variable holding the value of the power_savings_sd
> + * for cpu
> + */
> +#define for_each_powersaving_domain(cpu, sd) \
> +	for (sd = lowest_powersavings_sd(cpu); \
> +		(sd && (sd->flags & SD_POWERSAVINGS_BALANCE)); sd = sd->parent)

How about for_each_domain_flag(cpu, sd, flag) ?

(missing empty line)

> +/**
> + * find_new_ilb: Finds or nominates a new idle load balancer.
> + * @cpu: The cpu which is nominating a new idle_load_balancer.
> + *
> + * This algorithm picks the idle load balancer such that it belongs to a
> + * semi-idle powersavings sched_domain. The idea is to try and avoid
> + * completely idle packages/cores just for the purpose of idle load balancing
> + * when there are other idle cpu's which are better suited for that job.
> + */
> +static int find_new_ilb(int cpu)
> +{
> +	struct sched_domain *sd;
> +	struct sched_group *ilb_group;
> +	cpumask_t cpumask;

Mike Travis will hate you ;-)

> +	for_each_powersaving_domain(cpu, sd) {
> +		ilb_group = sd->groups;
> +
> +		do {
> +			cpus_and(cpumask, nohz.cpu_mask, ilb_group->cpumask);
> +
> +			/*
> +			 * If there exists atleast one busy cpu within
> +			 * this group, we should wake up the idle cpu from
> +			 * here.
> +			 */
> +			if (!(cpus_empty(cpumask) ||
> +				cpus_equal(cpumask, ilb_group->cpumask)))
> +				return first_cpu(cpumask);
> +
> +			ilb_group = ilb_group->next;
> +
> +		} while (ilb_group != sd->groups);
> +	}
> +	return first_cpu(nohz.cpu_mask);
> +}
> +#else /*  (CONFIG_SCHED_MC || CONFIG_SCHED_SMT) */
> +static inline int find_new_ilb(int call_cpu)
> +{
> +	return first_cpu(nohz.cpu_mask);
> +}
> +#endif
> +
>  /*
>   * Trigger the SCHED_SOFTIRQ if it is time to do periodic load balancing.
>   *
> @@ -3984,15 +4060,7 @@ static inline void trigger_load_balance(struct rq *rq, int cpu)
>  		}
>  
>  		if (atomic_read(&nohz.load_balancer) == -1) {
> -			/*
> -			 * simple selection for now: Nominate the
> -			 * first cpu in the nohz list to be the next
> -			 * ilb owner.
> -			 *
> -			 * TBD: Traverse the sched domains and nominate
> -			 * the nearest cpu in the nohz.cpu_mask.
> -			 */
> -			int ilb = first_cpu(nohz.cpu_mask);
> +			int ilb = find_new_ilb(cpu);
>  
>  			if (ilb < nr_cpu_ids)
>  				resched_cpu(ilb);

I guess so..