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

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Date:	Wed, 25 May 2016 18:29:33 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	yuyang.du@...el.com, Vincent Guittot <vincent.guittot@...aro.org>,
	Mike Galbraith <mgalbraith@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

2016-05-25 17:49 GMT+08:00 Morten Rasmussen <morten.rasmussen@....com>:
> On Wed, May 25, 2016 at 02:57:00PM +0800, Wanpeng Li wrote:
>> 2016-05-23 18:58 GMT+08:00 Morten Rasmussen <morten.rasmussen@....com>:
>> > 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;
>> > +
>> > +       return prev_cap * 1024 < task_util(p) * capacity_margin;
>> > +}
>>
>> If one task util_avg is SCHED_CAPACITY_SCALE and running on x86 box w/
>> SMT enabled, then each HT has capacity 589, wake_cap() will result in
>> always not wake affine, right?
>
> The idea is that SMT systems would bail out already at the previous
> condition. We should have max_cpu_capacity == prev_cap == 589, delta
> should then be zero and make the first condition true and make
> wake_cap() always return 0 for any system with symmetric capacities
> regardless of their actual capacity values.
>
> Note that this isn't entirely true as I used capacity_of() for prev_cap,
> if I change that to capacity_orig_of() it should be true.
>
> By making the !wake_cap() condition always true for want_affine, we
> should preserve existing behaviour for SMT/SMP. The only overhead is the
> capacity delta computation and comparison, which should be cheap.
>
> Does that make sense?

Fair enough, thanks for your explanation.

>
> Btw, task util_avg == SCHED_CAPACITY_SCALE should only be possible
> temporarily, it should decay to util_avg <=
> capacity_orig_of(task_cpu(p)) over time. That doesn't affect your

Sorry, I didn't find it will decay to capacity_orig in
__update_load_avg(), could you elaborate?

Regards,
Wanpeng Li