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

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Message-ID: <20160524081054.GE27946@e105550-lin.cambridge.arm.com>
Date:	Tue, 24 May 2016 09:10:55 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	peterz@...radead.org, mingo@...hat.com, dietmar.eggemann@....com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

On Tue, May 24, 2016 at 08:04:24AM +0800, Yuyang Du wrote:
> On Mon, May 23, 2016 at 11:58:51AM +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.
> > 
> > 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;
> 
> delta can be negative? still return 0?

I could add an abs() around delta.

Do you have a specific scenario in mind? Under normal circumstances, I
don't think it can be negative?