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

Morten Rasmussen
2016-05-24T08:10:18Z

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@redhat.com>
> > cc: Peter Zijlstra <peterz@infradead.org>
> >
> > Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.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?