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Message-Id: <1222167277.16700.73.camel@lappy.programming.kicks-ass.net>
Date: Tue, 23 Sep 2008 12:54:37 +0200
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
To: ego@...ibm.com
Cc: Ingo Molnar <mingo@...e.hu>,
Suresh Siddha <suresh.b.siddha@...el.com>,
linux-kernel@...r.kernel.org,
Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
Srivatsa Vaddagiri <vatsa@...ibm.com>,
Balbir Singh <balbir@...ibm.com>
Subject: Re: [PATCH] sched: Nominate the idle load balancer from a
semi-idle group.
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@...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@...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..
--
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