Re: [PATCH v2 07/13] sched/fair: Let asymmetric cpu configurations balance at wake-up

[Vincent Guittot]

On 13 July 2016 at 18:14, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
> On Wed, Jul 13, 2016 at 02:56:41PM +0200, Vincent Guittot wrote:
>> On 22 June 2016 at 19:03, Morten Rasmussen <morten.rasmussen@xxxxxxx> 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 all the cpu capacities
>> > available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup
>>
>> instead of "suitable for all the cpu capacities available within the
>> SD_WAKE_AFFINE sched_domain", should it be "suitable for local cpu and
>> prev cpu" becasue you only check the capacity of these 2 CPUs.
>
> Good point. I currently make the implicit assumption that capacity of local cpu
> and prev cpu represent the capacity for all cpus their SD_WAKE_AFFINE
> domains. It breaks if you should choose to have SD_WAKE_AFFINE on a
> domain that spans both little and big cpus, as if local/prev cpu happens
> to be big we assume that they are all big and let select_idle_sibling()
> handle the task placement even for big tasks if local/prev cpu are both
> big.

Isn't the sd_llc used in select_idle_sibling and not the
SD_WAKE_AFFINE domian so if CPUs in the sd_llc are homogeneous, we are
safe

>
> I don't see why anybody would want that kind of setup, but I think the
> assumption should still be written down somewhere, either here or in a
> comment in wake_cap() or both.
>
> The next paragraph in the commit message mentions that we actually only
> check waker cpu and prev_cpu capacity. Would it be more clear if we
> extend that to something like:
>
>     This patch makes affine wake-ups conditional on whether both the waker
>     cpu and prev_cpu has sufficient capacity for the waking task, or
>     not, assuming that the cpu capacities within an SD_WAKE_AFFINE
>     domain are homogeneous.
>
> Thoughts?
>
>>
>> Other than this comment for the commit message, the patch looks good to me
>> Acked-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>
> Thanks,
> Morten
>
>
>>
>> > balancing take over (find_idlest_{group, cpu}()).
>> >
>> > This patch makes affine wake-ups conditional on whether both the waker
>> > cpu and prev_cpu has sufficient capacity for the waking task, or not.
>> >
>> > It is assumed that the sched_group(s) containing the waker cpu and
>> > prev_cpu only contain cpu with the same capacity (homogeneous).
>> >
>> > 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@xxxxxxxxxx>
>> > cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> >
>> > Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
>> > ---
>> >  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 216db302e87d..dba02c7b57b3 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;
>> > @@ -5260,6 +5266,25 @@ 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 min_cap, max_cap;
>> > +
>> > +       min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));
>> > +       max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;
>> > +
>> > +       /* Minimum capacity is close to max, no need to abort wake_affine */
>> > +       if (max_cap - min_cap < max_cap >> 3)
>> > +               return 0;
>> > +
>> > +       return min_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,
>> > @@ -5283,7 +5308,8 @@ 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();
>> > --
>> > 1.9.1
>> >