Re: [PATCH 2/3] sched/fair: Introduce scaled capacity awareness in select_idle_sibling code path

[Rohit Jain]

On 09/25/2017 11:53 PM, Joel Fernandes wrote:
> Hi Rohit,
>
> Just some comments:

Hi Joel,

Thanks for the comments.

> On Mon, Sep 25, 2017 at 5:02 PM, Rohit Jain <rohit.k.jain@xxxxxxxxxx> wrote:
> > While looking for CPUs to place running tasks on, the scheduler
> > completely ignores the capacity stolen away by RT/IRQ tasks.
> >
> > This patch fixes that.
> >
> > Signed-off-by: Rohit Jain <rohit.k.jain@xxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 54 ++++++++++++++++++++++++++++++++++++++++++-----------
> >   1 file changed, 43 insertions(+), 11 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index afb701f..19ff2c3 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6040,7 +6040,10 @@ void __update_idle_core(struct rq *rq)
> >   static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
> >   {
> >          struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> > -       int core, cpu;
> > +       int core, cpu, rcpu, rcpu_backup;
> I would call rcpu_backup as backup_cpu.

OK

> > +       unsigned int backup_cap = 0;
> > +
> > +       rcpu = rcpu_backup = -1;
> >
> >          if (!static_branch_likely(&sched_smt_present))
> >                  return -1;
> > @@ -6057,10 +6060,20 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
> >                          cpumask_clear_cpu(cpu, cpus);
> >                          if (!idle_cpu(cpu))
> >                                  idle = false;
> > +
> > +                       if (full_capacity(cpu)) {
> > +                               rcpu = cpu;
> > +                       } else if ((rcpu == -1) && (capacity_of(cpu) > backup_cap)) {
> > +                               backup_cap = capacity_of(cpu);
> > +                               rcpu_backup = cpu;
> > +                       }
> Here you comparing capacity of different SMT threads.
>
> >                  }
> >
> > -               if (idle)
> > -                       return core;
> > +               if (idle) {
> > +                       if (rcpu == -1)
> > +                               return (rcpu_backup != -1 ? rcpu_backup : core);
> > +                       return rcpu;
> > +               }
>
> This didn't make much sense to me, here you are returning either an
> SMT thread or a core. That doesn't make much of a difference because
> SMT threads share the same capacity (SD_SHARE_CPUCAPACITY). I think
> what you want to do is find out the capacity of a 'core', not an SMT
> thread, and compare the capacity of different cores and consider the
> one which has least RT/IRQ interference.

IIUC the capacities of each strand is scaled by IRQ and 'rt_avg' for that
'rq'. Now if the strand is idle now and gets an interrupt in the future,
the 'core' would look like:

ÂÂ +----+----+
ÂÂ | IÂ |ÂÂÂ |
ÂÂ | TÂ |ÂÂÂ |
ÂÂ +----+----+

(I -> Interrupt, T-> Thread we are trying to schedule).

whereas if the other strand on the core was taking interrupt the core
would look like:

ÂÂ +----+----+
ÂÂ | IÂ | TÂ |
ÂÂ |ÂÂÂ |ÂÂÂ |
ÂÂ +----+----+

With this case, because we know from the past avg, one of the strands is
running low on capacity, I am trying to return a better strand for the
thread to start on.

> >          }
> >
> >          /*
> > @@ -6076,7 +6089,8 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
> >    */
> >   static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
> >   {
> > -       int cpu;
> > +       int cpu, backup_cpu = -1;
> > +       unsigned int backup_cap = 0;
> >
> >          if (!static_branch_likely(&sched_smt_present))
> >                  return -1;
> > @@ -6084,11 +6098,17 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
> >          for_each_cpu(cpu, cpu_smt_mask(target)) {
> >                  if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> >                          continue;
> > -               if (idle_cpu(cpu))
> > -                       return cpu;
> > +               if (idle_cpu(cpu)) {
> > +                       if (full_capacity(cpu))
> > +                               return cpu;
> > +                       if (capacity_of(cpu) > backup_cap) {
> > +                               backup_cap = capacity_of(cpu);
> > +                               backup_cpu = cpu;
> > +                       }
> > +               }
> Same thing here, since SMT threads share the same underlying capacity,
> is there any point in comparing the capacities of each SMT thread?

See above

Thanks,
Rohit

> thanks,
>
> - Joel
>
> [...]