Re: [PATCH v5 6/6] sched/fair: Consider SMT in ASYM_PACKING load balance

[Ricardo Neri]

On Wed, Sep 15, 2021 at 05:43:44PM +0200, Vincent Guittot wrote:
> On Sat, 11 Sept 2021 at 03:19, Ricardo Neri
> <ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
> >
> > When deciding to pull tasks in ASYM_PACKING, it is necessary not only to
> > check for the idle state of the destination CPU, dst_cpu, but also of
> > its SMT siblings.
> >
> > If dst_cpu is idle but its SMT siblings are busy, performance suffers
> > if it pulls tasks from a medium priority CPU that does not have SMT
> > siblings.
> >
> > Implement asym_smt_can_pull_tasks() to inspect the state of the SMT
> > siblings of both dst_cpu and the CPUs in the candidate busiest group.
> >
> > Cc: Aubrey Li <aubrey.li@xxxxxxxxx>
> > Cc: Ben Segall <bsegall@xxxxxxxxxx>
> > Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> > Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Cc: Quentin Perret <qperret@xxxxxxxxxx>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> > Changes since v4:
> >   * Use sg_lb_stats::sum_nr_running the idle state of a scheduling group.
> >     (Vincent, Peter)
> >   * Do not even idle CPUs in asym_smt_can_pull_tasks(). (Vincent)
> >   * Updated function documentation and corrected a typo.
> >
> > Changes since v3:
> >   * Removed the arch_asym_check_smt_siblings() hook. Discussions with the
> >     powerpc folks showed that this patch should not impact them. Also, more
> >     recent powerpc processor no longer use asym_packing. (PeterZ)
> >   * Removed unnecessary local variable in asym_can_pull_tasks(). (Dietmar)
> >   * Removed unnecessary check for local CPUs when the local group has zero
> >     utilization. (Joel)
> >   * Renamed asym_can_pull_tasks() as asym_smt_can_pull_tasks() to reflect
> >     the fact that it deals with SMT cases.
> >   * Made asym_smt_can_pull_tasks() return false for !CONFIG_SCHED_SMT so
> >     that callers can deal with non-SMT cases.
> >
> > Changes since v2:
> >   * Reworded the commit message to reflect updates in code.
> >   * Corrected misrepresentation of dst_cpu as the CPU doing the load
> >     balancing. (PeterZ)
> >   * Removed call to arch_asym_check_smt_siblings() as it is now called in
> >     sched_asym().
> >
> > Changes since v1:
> >   * Don't bailout in update_sd_pick_busiest() if dst_cpu cannot pull
> >     tasks. Instead, reclassify the candidate busiest group, as it
> >     may still be selected. (PeterZ)
> >   * Avoid an expensive and unnecessary call to cpumask_weight() when
> >     determining if a sched_group is comprised of SMT siblings.
> >     (PeterZ).
> > ---
> >  kernel/sched/fair.c | 94 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 94 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 26db017c14a3..8d763dd0174b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8597,10 +8597,98 @@ group_type group_classify(unsigned int imbalance_pct,
> >         return group_has_spare;
> >  }
> >
> > +/**
> > + * 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 busiest 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, and @sg has exactly one more
> > + * busy CPU than @sds::local, let @dst_cpu pull tasks if it has higher priority.
> > + * Bigger imbalances in the number of busy CPUs will be dealt with in
> > + * update_sd_pick_busiest().
> > + *
> > + * 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)
> 
> Do you really need to test sg_is_smt ? if sg_busy_cpus >= 2 then
> sd_is_smt must be true ?

Thank you very much for your feedback Vincent!

Yes, it is true that sg_busy_cpus >=2 is only true if @sg is SMT. I will
remove this check.

> 
> Also, This is the default behavior where we want to even the number of
> busy cpu. Shouldn't you return false and fall back to the default
> behavior ?

This is also true.

> 
> That being said, the default behavior tries to even the number of idle
> cpus which is easier to compute and is equal to even the number of
> busy cpus in "normal" system with the same number of cpus in groups
> but this is not the case here. It could be good to change the default
> behavior to even the number of busy cpus and that you use the default
> behavior here. Additional condition will be used to select the busiest
> group like more busy cpu or more number of running tasks

That is a very good observation. Checking the number of idle CPUs
assumes that both groups have the same number of CPUs. I'll look into
modifying the default behavior.

> 
> > +                       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 (i.e.,
> > +                * it has no running tasks).
> 
> The previous comment above assume that "@dst_cpu is idle" but now you
> need to check that sds->local_stat.sum_nr_running == 0

But we already know that, right? We are here because in
update_sg_lb_stats() we determine that dst CPU is idle (env->idle !=
CPU_NOT_IDLE).

Thanks and BR,
Ricardo