Re: [PATCH v4 6/6] sched/fair: Consider SMT in ASYM_PACKING load balance

[Ricardo Neri]

On Fri, Aug 27, 2021 at 05:17:22PM +0200, Vincent Guittot wrote:
> On Fri, 27 Aug 2021 at 16:50, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Aug 27, 2021 at 12:13:42PM +0200, Vincent Guittot wrote:
> > > > +/**
> > > > + * asym_smt_can_pull_tasks - Check whether the load balancing CPU can pull tasks
> > > > + * @dst_cpu:   Destination CPU of the load balancing
> > > > + * @sds:       Load-balancing data with statistics of the local group
> > > > + * @sgs:       Load-balancing statistics of the candidate busiest group
> > > > + * @sg:                The candidate busiet group
> > > > + *
> > > > + * Check the state of the SMT siblings of both @sds::local and @sg and decide
> > > > + * if @dst_cpu can pull tasks. If @dst_cpu does not have SMT siblings, it can
> > > > + * pull tasks if two or more of the SMT siblings of @sg are busy. If only one
> > > > + * CPU in @sg is busy, pull tasks only if @dst_cpu has higher priority.
> > > > + *
> > > > + * If both @dst_cpu and @sg have SMT siblings, even the number of idle CPUs
> > > > + * between @sds::local and @sg. Thus, pull tasks from @sg if the difference
> > > > + * between the number of busy CPUs is 2 or more. If the difference is of 1,
> > > > + * only pull if @dst_cpu has higher priority. If @sg does not have SMT siblings
> > > > + * only pull tasks if all of the SMT siblings of @dst_cpu are idle and @sg
> > > > + * has lower priority.
> > > > + */
> > > > +static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
> > > > +                                   struct sg_lb_stats *sgs,
> > > > +                                   struct sched_group *sg)
> > > > +{
> > > > +#ifdef CONFIG_SCHED_SMT
> > > > +       bool local_is_smt, sg_is_smt;
> > > > +       int sg_busy_cpus;
> > > > +
> > > > +       local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;
> > > > +       sg_is_smt = sg->flags & SD_SHARE_CPUCAPACITY;
> > > > +
> > > > +       sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;
> > > > +
> > > > +       if (!local_is_smt) {
> > > > +               /*
> > > > +                * If we are here, @dst_cpu is idle and does not have SMT
> > > > +                * siblings. Pull tasks if candidate group has two or more
> > > > +                * busy CPUs.
> > > > +                */
> > > > +               if (sg_is_smt && sg_busy_cpus >= 2)
> > > > +                       return true;
> > > > +
> > > > +               /*
> > > > +                * @dst_cpu does not have SMT siblings. @sg may have SMT
> > > > +                * siblings and only one is busy. In such case, @dst_cpu
> > > > +                * can help if it has higher priority and is idle.
> > > > +                */
> > > > +               return !sds->local_stat.group_util &&
> > >
> > > sds->local_stat.group_util can't be used to decide if a CPU or group
> > > of CPUs is idle. util_avg is usually not null when a CPU becomes idle
> > > and you can have to wait  more than 300ms before it becomes Null
> > > At the opposite, the utilization of a CPU can be null but a task with
> > > null utilization has just woken up on it.
> > > Utilization is used to reflect the average work of the CPU or group of
> > > CPUs but not the current state
> >
> > If you want immediate idle, sgs->nr_running == 0 or sgs->idle_cpus ==
> > sgs->group_weight come to mind.
> 
> yes, I have the same in mind

Thank you very much Vincent and Peter for the feedback! I'll look at
using these stats to determine immediate idle.

> 
> >
> > > > +                      sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
> > > > +       }
> > > > +
> > > > +       /* @dst_cpu has SMT siblings. */
> > > > +
> > > > +       if (sg_is_smt) {
> > > > +               int local_busy_cpus = sds->local->group_weight -
> > > > +                                     sds->local_stat.idle_cpus;
> > > > +               int busy_cpus_delta = sg_busy_cpus - local_busy_cpus;
> > > > +
> > > > +               /* Local can always help to even the number busy CPUs. */
> > >
> > > default behavior of the load balance already tries to even the number
>  a> > of idle CPUs.
> >
> > Right, but I suppose this is because we're trapped here and have to deal
> > with the SMT-SMT case too. Ricardo, can you clarify?
> 
> IIUC, this function is used in sg_lb_stats to set
> sgs->group_asym_packing which is then used to set the group state to
> group_asym_packing and force asym migration.
> But if we only want to even the number of busy CPUs between the group,
> we should not need to set the group state to  group_asym_packing

Yes, what Vincent describe is the intent. Then, I think it is probably
true that it is not necessary to even the number of idle CPUs here.
> 
> >
> > > > +               if (busy_cpus_delta >= 2)
> > > > +                       return true;
> > > > +
> > > > +               if (busy_cpus_delta == 1)
> > > > +                       return sched_asym_prefer(dst_cpu,
> > > > +                                                sg->asym_prefer_cpu);

I think we should keep this check in order to move tasks to higher
priority CPUs.

> > > > +
> > > > +               return false;
> > > > +       }
> > > > +
> > > > +       /*
> > > > +        * @sg does not have SMT siblings. Ensure that @sds::local does not end
> > > > +        * up with more than one busy SMT sibling and only pull tasks if there
> > > > +        * are not busy CPUs. As CPUs move in and out of idle state frequently,
> > > > +        * also check the group utilization to smoother the decision.
> > > > +        */
> > > > +       if (!sds->local_stat.group_util)
> > >
> > > same comment as above about the meaning of group_util == 0

I will look into using the suggested statistics.

Thanks and BR,
Ricardo