Re: [PATCH 2/2] sched/fair: Relax task_hot() for misfit tasks

[Vincent Guittot <vincent.guittot@linaro.org>]

On Mon, 19 Apr 2021 at 19:13, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> On 16/04/21 15:51, Vincent Guittot wrote:
> > Le jeudi 15 avril 2021 � 18:58:46 (+0100), Valentin Schneider a �crit :
> >> +
> >> +/*
> >> + * What does migrating this task do to our capacity-aware scheduling criterion?
> >> + *
> >> + * Returns 1, if the task needs more capacity than the dst CPU can provide.
> >> + * Returns 0, if the task needs the extra capacity provided by the dst CPU
> >> + * Returns -1, if the task isn't impacted by the migration wrt capacity.
> >> + */
> >> +static int migrate_degrades_capacity(struct task_struct *p, struct lb_env *env)
> >> +{
> >> +    if (!(env->sd->flags & SD_ASYM_CPUCAPACITY))
> >> +            return -1;
> >> +
> >> +    if (!task_fits_capacity(p, capacity_of(env->src_cpu))) {
> >> +            if (cpu_capacity_greater(env->dst_cpu, env->src_cpu))
> >> +                    return 0;
> >> +            else if (cpu_capacity_greater(env->src_cpu, env->dst_cpu))
> >> +                    return 1;
> >> +            else
> >> +                    return -1;
> >> +    }
> >
> > Being there means that task fits src_cpu capacity so why testing p against dst_cpu ?
> >
>
> Because if p fits on src_cpu, we don't want to move it to a dst_cpu on
> which it *doesn't* fit.

OK. I was confused because I thought that this was only to force
migration in case of group_misfit_task but you tried to extend to
other cases... I'm not convinced that you succeeded to cover all cases

Also I found this function which returns 3 values a bit disturbing.
IIUC you tried to align to migrate_degrades_capacity but you should
have better aligned to task_hot and return only 0 or 1. -1 is not used

>
> >> +
> >> +    return task_fits_capacity(p, capacity_of(env->dst_cpu)) ? -1 : 1;
> >> +}
> >
> > I prefer the below which easier to read because the same var is use everywhere and you can remove cpu_capacity_greater.
> >
> > static int migrate_degrades_capacity(struct task_struct *p, struct lb_env *env)
> > {
> >     unsigned long src_capacity, dst_capacity;
> >
> >     if (!(env->sd->flags & SD_ASYM_CPUCAPACITY))
> >         return -1;
> >
> >     src_capacity = capacity_of(env->src_cpu);
> >     dst_capacity = capacity_of(env->dst_cpu);
> >
> >     if (!task_fits_capacity(p, src_capacity)) {
> >         if (capacity_greater(dst_capacity, src_capacity))
> >             return 0;
> >         else if (capacity_greater(src_capacity, dst_capacity))
> >             return 1;
> >         else
> >             return -1;
> >     }
> >
> >     return task_fits_capacity(p, dst_capacity) ? -1 : 1;
> > }
> >
>
> I'll take it, thanks!
>
> >
> >> +
> >>  #ifdef CONFIG_NUMA_BALANCING
> >>  /*
> >>   * Returns 1, if task migration degrades locality
> >> @@ -7672,6 +7698,15 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >>      if (tsk_cache_hot == -1)
> >>              tsk_cache_hot = task_hot(p, env);
> >>
> >> +    /*
> >> +     * On a (sane) asymmetric CPU capacity system, the increase in compute
> >> +     * capacity should offset any potential performance hit caused by a
> >> +     * migration.
> >> +     */
> >> +    if ((env->dst_grp_type == group_has_spare) &&
> >
> > Shouldn't it be env->src_grp_type == group_misfit_task to only care of misfit task case as
> > stated in $subject
> >
>
> Previously this was env->idle != CPU_NOT_IDLE, but I figured dst_grp_type
> could give us a better picture. Staring at this some more, this isn't so
> true when the group size goes up - there's no guarantees the dst_cpu is the
> one that has spare cycles, and the other CPUs might not be able to grant
> the capacity uplift dst_cpu can.

yeah you have to keep checking for env->idle != CPU_NOT_IDLE

>
> As for not using src_grp_type == group_misfit_task, this is pretty much the
> same as [1]. CPU-bound (misfit) task + some other task on the same rq
> implies group_overloaded classification when balancing at MC level (no SMT,
> so one group per CPU).

Is it something that happens often or just a sporadic/transient state
? I mean does it really worth the extra complexity and do you see
performance improvement ?

You should better focus on fixing the simple case of group_misfit_task
task. This other cases looks far more complex with lot of corner cases

>
> [1]: http://lore.kernel.org/r/jhjblcuv2mo.mognet@arm.com