Re: [PATCH v3 10/24] sched/fair: Use IPCC scores to select a busiest runqueue

[Vincent Guittot]

On Tue, 7 Feb 2023 at 06:01, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
>
> For two runqueues of equal priority and equal number of running of tasks,
> select the one whose current task would have the highest IPC class score
> if placed on the destination CPU.

You failed to explain why it make sense to compare current task score
whereas we will most probably not pull this task at the end
>
> For now, use IPCC scores only for scheduling domains with the
> SD_ASYM_PACKING flag.
>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Cc: Len Brown <len.brown@xxxxxxxxx>
> Cc: Lukasz Luba <lukasz.luba@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Tim C. Chen <tim.c.chen@xxxxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-pm@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since v2:
>  * Only use IPCC scores to break ties if the sched domain uses
>    asym_packing. (Ionela)
>  * Handle errors of arch_get_ipcc_score(). (Ionela)
>
> Changes since v1:
>  * Fixed a bug when selecting a busiest runqueue: when comparing two
>    runqueues with equal nr_running, we must compute the IPCC score delta
>    of both.
>  * Renamed local variables to improve the layout of the code block.
>    (PeterZ)
>  * Used the new interface names.
> ---
>  kernel/sched/fair.c | 64 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 72d88270b320..d3c22dc145f7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9399,6 +9399,37 @@ static bool sched_asym_ipcc_pick(struct sched_group *a,
>         return sched_asym_ipcc_prefer(a_stats, b_stats);
>  }
>
> +/**
> + * ipcc_score_delta - Get the IPCC score delta wrt the load balance's dst_cpu
> + * @p:         A task
> + * @env:       Load balancing environment
> + *
> + * Returns: The IPCC score delta that @p would get if placed in the destination
> + * CPU of @env. LONG_MIN to indicate that the delta should not be used.
> + */
> +static long ipcc_score_delta(struct task_struct *p, struct lb_env *env)
> +{
> +       unsigned long score_src, score_dst;
> +       unsigned short ipcc = p->ipcc;
> +
> +       if (!sched_ipcc_enabled())
> +               return LONG_MIN;
> +
> +       /* Only asym_packing uses IPCC scores at the moment. */
> +       if (!(env->sd->flags & SD_ASYM_PACKING))
> +               return LONG_MIN;
> +
> +       score_dst = arch_get_ipcc_score(ipcc, env->dst_cpu);
> +       if (IS_ERR_VALUE(score_dst))
> +               return LONG_MIN;
> +
> +       score_src = arch_get_ipcc_score(ipcc, task_cpu(p));
> +       if (IS_ERR_VALUE(score_src))
> +               return LONG_MIN;
> +
> +       return score_dst - score_src;
> +}
> +
>  #else /* CONFIG_IPC_CLASSES */
>  static void update_sg_lb_ipcc_stats(int dst_cpu, struct sg_lb_stats *sgs,
>                                     struct rq *rq)
> @@ -9429,6 +9460,11 @@ static bool sched_asym_ipcc_pick(struct sched_group *a,
>         return false;
>  }
>
> +static long ipcc_score_delta(struct task_struct *p, struct lb_env *env)
> +{
> +       return LONG_MIN;
> +}
> +
>  #endif /* CONFIG_IPC_CLASSES */
>
>  /**
> @@ -10589,6 +10625,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>  {
>         struct rq *busiest = NULL, *rq;
>         unsigned long busiest_util = 0, busiest_load = 0, busiest_capacity = 1;
> +       long busiest_ipcc_delta = LONG_MIN;
>         unsigned int busiest_nr = 0;
>         int i;
>
> @@ -10705,8 +10742,35 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>
>                 case migrate_task:
>                         if (busiest_nr < nr_running) {
> +                               struct task_struct *curr;
> +
>                                 busiest_nr = nr_running;
>                                 busiest = rq;
> +
> +                               /*
> +                                * Remember the IPCC score delta of busiest::curr.
> +                                * We may need it to break a tie with other queues
> +                                * with equal nr_running.
> +                                */
> +                               curr = rcu_dereference(busiest->curr);
> +                               busiest_ipcc_delta = ipcc_score_delta(curr, env);

Hmm, i don't like this at all

Also, curr is the least probable task to be pulled which means that
all this his useless

> +                       /*
> +                        * If rq and busiest have the same number of running
> +                        * tasks and IPC classes are supported, pick rq if doing
> +                        * so would give rq::curr a bigger IPC boost on dst_cpu.
> +                        */
> +                       } else if (busiest_nr == nr_running) {
> +                               struct task_struct *curr;
> +                               long delta;
> +
> +                               curr = rcu_dereference(rq->curr);
> +                               delta = ipcc_score_delta(curr, env);
> +
> +                               if (busiest_ipcc_delta < delta) {
> +                                       busiest_ipcc_delta = delta;
> +                                       busiest_nr = nr_running;
> +                                       busiest = rq;
> +                               }
>                         }
>                         break;
>
> --
> 2.25.1
>