Re: [PATCH v9 05/10] sched: make scale_rt invariant with frequency

[Wanpeng Li]

Hi Vincent,
On 11/25/14, 9:52 PM, Vincent Guittot wrote:
> On 25 November 2014 at 03:24, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> > Hi Vincent,
> > On 11/4/14, 12:54 AM, Vincent Guittot wrote:
> > > The average running time of RT tasks is used to estimate the remaining
> > > compute
> > > capacity for CFS tasks. This remaining capacity is the original capacity
> > > scaled
> > > down by a factor (aka scale_rt_capacity). This estimation of available
> > > capacity
> > > must also be invariant with frequency scaling.
> > >
> > > A frequency scaling factor is applied on the running time of the RT tasks
> > > for
> > > computing scale_rt_capacity.
> > >
> > > In sched_rt_avg_update, we scale the RT execution time like below:
> > > rq->rt_avg += rt_delta * arch_scale_freq_capacity() >>
> > > SCHED_CAPACITY_SHIFT
> > >
> > > Then, scale_rt_capacity can be summarized by:
> > > scale_rt_capacity = SCHED_CAPACITY_SCALE -
> > >                  ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)
> >
> > The 'period' aka 'total' in the scale_rt_capacity(), why it is
> > sched_avg_period() + delta instead of sched_avg_period()?
> The default value of sched_avg_period is 1sec which is "long" so we
> take into account the time consumed by RT tasks in the ongoing period
> .

Do you mean 'sched_avg_period() + delta' should be replaced by 'delta' 
since sched_avg_period() is "long"?

Regards,
Wanpeng Li

> > Regards,
> > Wanpeng Li
> >
> >
> > > We can optimize by removing right and left shift in the computation of
> > > rq->rt_avg
> > > and scale_rt_capacity
> > >
> > > The call to arch_scale_frequency_capacity in the rt scheduling path might
> > > be
> > > a concern for RT folks because I'm not sure whether we can rely on
> > > arch_scale_freq_capacity to be short and efficient ?
> > >
> > > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > > ---
> > >    kernel/sched/fair.c  | 17 +++++------------
> > >    kernel/sched/sched.h |  4 +++-
> > >    2 files changed, 8 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index a5039da..b37c27b 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -5785,7 +5785,7 @@ unsigned long __weak arch_scale_cpu_capacity(struct
> > > sched_domain *sd, int cpu)
> > >    static unsigned long scale_rt_capacity(int cpu)
> > >    {
> > >          struct rq *rq = cpu_rq(cpu);
> > > -       u64 total, available, age_stamp, avg;
> > > +       u64 total, used, age_stamp, avg;
> > >          s64 delta;
> > >          /*
> > > @@ -5801,19 +5801,12 @@ static unsigned long scale_rt_capacity(int cpu)
> > >          total = sched_avg_period() + delta;
> > >    -     if (unlikely(total < avg)) {
> > > -               /* Ensures that capacity won't end up being negative */
> > > -               available = 0;
> > > -       } else {
> > > -               available = total - avg;
> > > -       }
> > > +       used = div_u64(avg, total);
> > >    -     if (unlikely((s64)total < SCHED_CAPACITY_SCALE))
> > > -               total = SCHED_CAPACITY_SCALE;
> > > +       if (likely(used < SCHED_CAPACITY_SCALE))
> > > +               return SCHED_CAPACITY_SCALE - used;
> > >    -     total >>= SCHED_CAPACITY_SHIFT;
> > > -
> > > -       return div_u64(available, total);
> > > +       return 1;
> > >    }
> > >      static void update_cpu_capacity(struct sched_domain *sd, int cpu)
> > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > index c34bd11..fc5b152 100644
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -1312,9 +1312,11 @@ static inline int hrtick_enabled(struct rq *rq)
> > >      #ifdef CONFIG_SMP
> > >    extern void sched_avg_update(struct rq *rq);
> > > +extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd,
> > > int cpu);
> > > +
> > >    static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
> > >    {
> > > -       rq->rt_avg += rt_delta;
> > > +       rq->rt_avg += rt_delta * arch_scale_freq_capacity(NULL,
> > > cpu_of(rq));
> > >          sched_avg_update(rq);
> > >    }
> > >    #else

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