On Thu, 15 Apr 2021 at 19:58, Valentin Schneider
<valentin.schnei...@arm.com> wrote:
>
> Consider the following (hypothetical) asymmetric CPU capacity topology,
> with some amount of capacity pressure (RT | DL | IRQ | thermal):
>
> DIE [ ]
> MC [ ][ ]
> 0 1 2 3
>
> | CPU | capacity_orig | capacity |
> |-----+---------------+----------|
> | 0 | 870 | 860 |
> | 1 | 870 | 600 |
> | 2 | 1024 | 850 |
> | 3 | 1024 | 860 |
>
> If CPU1 has a misfit task, then CPU0, CPU2 and CPU3 are valid candidates to
> grant the task an uplift in CPU capacity. Consider CPU0 and CPU3 as
> sufficiently busy, i.e. don't have enough spare capacity to accommodate
> CPU1's misfit task. This would then fall on CPU2 to pull the task.
>
> This currently won't happen, because CPU2 will fail
>
> capacity_greater(capacity_of(CPU2), sg->sgc->max_capacity)
>
> in update_sd_pick_busiest(), where 'sg' is the [0, 1] group at DIE
> level. In this case, the max_capacity is that of CPU0's, which is at this
> point in time greater than that of CPU2's. This comparison doesn't make
> much sense, given that the only CPUs we should care about in this scenario
> are CPU1 (the CPU with the misfit task) and CPU2 (the load-balance
> destination CPU).
>
> Aggregate a misfit task's load into sgs->group_misfit_task_load only if
> env->dst_cpu would grant it a capacity uplift.
>
> Note that the aforementioned capacity vs sgc->max_capacity comparison was
> meant to prevent misfit task downmigration: candidate groups classified as
> group_misfit_task but with a higher (max) CPU capacity than the destination
> CPU
> would be discarded. This change makes it so said group_misfit_task
> classification can't happen anymore, which may cause some undesired
> downmigrations.
>
> Further tweak find_busiest_queue() to ensure this doesn't happen. Also note
> find_busiest_queue() can now iterate over CPUs with a higher capacity than
> the local CPU's, so add a capacity check there.
>
> Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com>
> ---
> kernel/sched/fair.c | 63 ++++++++++++++++++++++++++++++++-------------
> 1 file changed, 45 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9b8ae02f1994..d2d1a69d7aa7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5759,6 +5759,12 @@ static unsigned long capacity_of(int cpu)
> return cpu_rq(cpu)->cpu_capacity;
> }
>
> +/* Is CPU a's capacity noticeably greater than CPU b's? */
> +static inline bool cpu_capacity_greater(int a, int b)
> +{
> + return capacity_greater(capacity_of(a), capacity_of(b));
> +}
> +
> static void record_wakee(struct task_struct *p)
> {
> /*
> @@ -7486,6 +7492,7 @@ struct lb_env {
>
> enum fbq_type fbq_type;
> enum migration_type migration_type;
> + enum group_type src_grp_type;
> struct list_head tasks;
> };
>
> @@ -8447,6 +8454,32 @@ static bool update_nohz_stats(struct rq *rq)
> #endif
> }
>
> +static inline void update_sg_lb_misfit_stats(struct lb_env *env,
> + struct sched_group *group,
> + struct sg_lb_stats *sgs,
> + int *sg_status,
> + int cpu)
> +{
> + struct rq *rq = cpu_rq(cpu);
> +
> + if (!(env->sd->flags & SD_ASYM_CPUCAPACITY) ||
> + !rq->misfit_task_load)
> + return;
> +
> + *sg_status |= SG_OVERLOAD;
> +
> + /*
> + * Don't attempt to maximize load for misfit tasks that can't be
> + * granted a CPU capacity uplift.
> + */
> + if (cpu_capacity_greater(env->dst_cpu, cpu)) {
> + sgs->group_misfit_task_load = max(
> + sgs->group_misfit_task_load,
> + rq->misfit_task_load);
> + }
> +
> +}
> +
> /**
> * update_sg_lb_stats - Update sched_group's statistics for load balancing.
> * @env: The load balancing environment.
> @@ -8498,12 +8531,7 @@ static inline void update_sg_lb_stats(struct lb_env
> *env,
> if (local_group)
> continue;
>
> - /* Check for a misfit task on the cpu */
> - if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> - sgs->group_misfit_task_load < rq->misfit_task_load) {
> - sgs->group_misfit_task_load = rq->misfit_task_load;
> - *sg_status |= SG_OVERLOAD;
> - }
> + update_sg_lb_misfit_stats(env, group, sgs, sg_status, i);
> }
>
> /* Check if dst CPU is idle and preferred to this group */
> @@ -8550,15 +8578,9 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> if (!sgs->sum_h_nr_running)
> return false;
>
> - /*
> - * Don't try to pull misfit tasks we can't help.
> - * We can use max_capacity here as reduction in capacity on some
> - * CPUs in the group should either be possible to resolve
> - * internally or be covered by avg_load imbalance (eventually).
> - */
> + /* Don't try to pull misfit tasks we can't help */
> if (sgs->group_type == group_misfit_task &&
> - (!capacity_greater(capacity_of(env->dst_cpu),
> sg->sgc->max_capacity) ||
> - sds->local_stat.group_type != group_has_spare))
> + sds->local_stat.group_type != group_has_spare)
> return false;
>
> if (sgs->group_type > busiest->group_type)
> @@ -9288,6 +9310,8 @@ static struct sched_group *find_busiest_group(struct
> lb_env *env)
> if (!sds.busiest)
> goto out_balanced;
>
> + env->src_grp_type = busiest->group_type;
> +
> /* Misfit tasks should be dealt with regardless of the avg load */
> if (busiest->group_type == group_misfit_task)
> goto force_balance;
> @@ -9441,8 +9465,8 @@ static struct rq *find_busiest_queue(struct lb_env *env,
> * average load.
> */
> if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
> - !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
> - nr_running == 1)
> + env->src_grp_type <= group_fully_busy &&
> + !capacity_greater(capacity_of(env->dst_cpu), capacity))
> continue;
>
> switch (env->migration_type) {
> @@ -9504,15 +9528,18 @@ static struct rq *find_busiest_queue(struct lb_env
> *env,
> case migrate_misfit:
> /*
> * For ASYM_CPUCAPACITY domains with misfit tasks we
> - * simply seek the "biggest" misfit task.
> + * simply seek the "biggest" misfit task we can
> + * accommodate.
> */
> + if (!cpu_capacity_greater(env->dst_cpu, i))
Use the same level of interface as above. This makes code and the
condition easier to follow in find_busiest_queue()
capacity_greater(capacity_of(env->dst_cpu), capacity_of(i))
> + continue;
> +
> if (rq->misfit_task_load > busiest_load) {
> busiest_load = rq->misfit_task_load;
> busiest = rq;
> }
>
> break;
> -
> }
> }
>
> --
> 2.25.1
>
