Re: [PATCH 6/8] sched/fair: Filter out locally-unsolvable misfit imbalances

[Qais Yousef <qais.yousef@arm.com>]

On 01/28/21 18:31, Valentin Schneider 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.

I think this scenario would be hard in practice, but not impossible. Maybe
gaming could push the system that hard.

> 
> 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. Separately track whether a
> sched_group contains a misfit task to still classify it as
> group_misfit_task and not pick it as busiest group when pulling from a
> lower-capacity CPU (which is the current behaviour and prevents
> down-migration).
> 
> Since find_busiest_queue() can now iterate over CPUs with a higher capacity
> than the local CPU's, add a capacity check there.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  kernel/sched/fair.c | 40 ++++++++++++++++++++++++++++++----------
>  1 file changed, 30 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ef44474b8fbf..0ac2f876b86f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5765,6 +5765,12 @@ 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)
>  {
>  	/*
> @@ -8093,7 +8099,8 @@ struct sg_lb_stats {
>  	unsigned int group_weight;
>  	enum group_type group_type;
>  	unsigned int group_asym_packing; /* Tasks should be moved to preferred CPU */
> -	unsigned long group_misfit_task_load; /* A CPU has a task too big for its capacity */
> +	unsigned long group_misfit_task_load; /* Task load that can be uplifted */
> +	int           group_has_misfit_task; /* A CPU has a task too big for its capacity */
>  #ifdef CONFIG_NUMA_BALANCING
>  	unsigned int nr_numa_running;
>  	unsigned int nr_preferred_running;
> @@ -8364,7 +8371,7 @@ group_type group_classify(unsigned int imbalance_pct,
>  	if (sgs->group_asym_packing)
>  		return group_asym_packing;
>  
> -	if (sgs->group_misfit_task_load)
> +	if (sgs->group_has_misfit_task)
>  		return group_misfit_task;
>  
>  	if (!group_has_capacity(imbalance_pct, sgs))
> @@ -8450,11 +8457,21 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  			continue;
>  
>  		/* Check for a misfit task on the cpu */
> -		if (sd_has_asym_cpucapacity(env->sd) &&
> -		    sgs->group_misfit_task_load < rq->misfit_task_load) {
> -			sgs->group_misfit_task_load = rq->misfit_task_load;
> -			*sg_status |= SG_OVERLOAD;
> -		}
> +		if (!sd_has_asym_cpucapacity(env->sd) ||
> +		    !rq->misfit_task_load)
> +			continue;
> +
> +		*sg_status |= SG_OVERLOAD;
> +		sgs->group_has_misfit_task = true;
> +
> +		/*
> +		 * 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, i))
> +			sgs->group_misfit_task_load = max(
> +				sgs->group_misfit_task_load,
> +				rq->misfit_task_load);

nit: missing curly braces around the if.

>  	}
>  
>  	/* Check if dst CPU is idle and preferred to this group */
> @@ -8504,7 +8521,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  	/* Don't try to pull misfit tasks we can't help */
>  	if (static_branch_unlikely(&sched_asym_cpucapacity) &&
>  	    sgs->group_type == group_misfit_task &&
> -	    (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
> +	    (!sgs->group_misfit_task_load ||
>  	     sds->local_stat.group_type != group_has_spare))
>  		return false;
>  
> @@ -9464,15 +9481,18 @@ 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))
> +				continue;

Both this hunk and the one above mean we will end up searching harder to pull
the task into the right cpu taking actual capacity into account. Which is
a good improvement.

Reviewed-by: Qais Yousef <qais.youesf@arm.com>

Thanks

--
Qais Yousef

> +
>  			if (rq->misfit_task_load > busiest_load) {
>  				busiest_load = rq->misfit_task_load;
>  				busiest = rq;
>  			}
>  
>  			break;
> -
>  		}
>  	}
>  
> -- 
> 2.27.0
>