linux-kernel - Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations balance at wake-up

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Date:	Tue, 24 May 2016 09:03:57 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Morten Rasmussen <morten.rasmussen@....com>, peterz@...radead.org,
	mingo@...hat.com
Cc:	dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

On Mon, 2016-05-23 at 11:58 +0100, Morten Rasmussen wrote:
> Currently, SD_WAKE_AFFINE always takes priority over wakeup balancing if
> SD_BALANCE_WAKE is set on the sched_domains. For asymmetric
> configurations SD_WAKE_AFFINE is only desirable if the waking task's
> compute demand (utilization) is suitable for the cpu capacities
> available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup
> balancing take over (find_idlest_{group, cpu}()).
> 
> The assumption is that SD_WAKE_AFFINE is never set for a sched_domain
> containing cpus with different capacities. This is enforced by a
> previous patch based on the SD_ASYM_CPUCAPACITY flag.
> 
> Ideally, we shouldn't set 'want_affine' in the first place, but we don't
> know if SD_BALANCE_WAKE is enabled on the sched_domain(s) until we start
> traversing them.

This doesn't look like it's restricted to big/little setups, so could
overrule wake_wide() wanting to NAK a x-node pull.

> > 
> > cc: Ingo Molnar <mingo@...hat.com>
> > cc: Peter Zijlstra <peterz@...radead.org>
> > 
> > Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> > ---
> >  kernel/sched/fair.c | 28 +++++++++++++++++++++++++++-
> >  1 file changed, 27 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 564215d..ce44fa7 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -114,6 +114,12 @@ unsigned int __read_mostly sysctl_sched_shares_window = 10000000UL;
> >  unsigned int sysctl_sched_cfs_bandwidth_slice = 5000UL;
> >  #endif
> >  
> > +/*
> > + * The margin used when comparing utilization with cpu capacity:
> > + * util * 1024 < capacity * margin
> > + */
> > +unsigned int capacity_margin = 1280; /* ~20% */
> > +
> >  static inline void update_load_add(struct load_weight *lw, unsigned long inc)
> >  {
> >  > > 	> > lw->weight += inc;
> > @@ -5293,6 +5299,25 @@ static int cpu_util(int cpu)
> >  > > 	> > return (util >= capacity) ? capacity : util;
> >  }
> >  
> > +static inline int task_util(struct task_struct *p)
> > +{
> > +> > 	> > return p->se.avg.util_avg;
> > +}
> > +
> > +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> > +{
> > +> > 	> > long delta;
> > +> > 	> > long prev_cap = capacity_of(prev_cpu);
> > +
> > +> > 	> > delta = cpu_rq(cpu)->rd->max_cpu_capacity - prev_cap;
> > +
> > +> > 	> > /* prev_cpu is fairly close to max, no need to abort wake_affine */
> > +> > 	> > if (delta < prev_cap >> 3)
> > +> > 	> > 	> > return 0;
> > +
> > +> > 	> > return prev_cap * 1024 < task_util(p) * capacity_margin;
> > +}
> > +
> >  /*
> >   * select_task_rq_fair: Select target runqueue for the waking task in domains
> >   * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> > @@ -5316,7 +5341,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> >  
> >  > > 	> > if (sd_flag & SD_BALANCE_WAKE) {
> >  > > 	> > 	> > record_wakee(p);
> > -> > 	> > 	> > want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> > +> > 	> > 	> > want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> > +> > 	> > 	> > 	> >       && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> >  > > 	> > }
> >  
> >  > > 	> > rcu_read_lock();