Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains

[Valentin Schneider <valentin.schneider@arm.com>]

On 18/12/2019 22:50, Mel Gorman wrote:
>> I'm quite sure you have reasons to have written it that way, but I was
>> hoping we could squash it down to something like:
> 
> I wrote it that way to make it clear exactly what has changed, the
> thinking behind the checks and to avoid 80-col limits to make review
> easier overall. It's a force of habit and I'm happy to reformat it as
> you suggest except....
> 

I tend to disregard the 80 col limit, so I might not be the best example
here :D

>> ---
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 08a233e97a01..f05d09a8452e 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -8680,16 +8680,27 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>>  			env->migration_type = migrate_task;
>>  			lsub_positive(&nr_diff, local->sum_nr_running);
>>  			env->imbalance = nr_diff >> 1;
>> -			return;
>> +		} else {
>> +
>> +			/*
>> +			 * If there is no overload, we just want to even the number of
>> +			 * idle cpus.
>> +			 */
>> +			env->migration_type = migrate_task;
>> +			env->imbalance = max_t(long, 0, (local->idle_cpus -
>> +							 busiest->idle_cpus) >> 1);
>>  		}
>>  
>>  		/*
>> -		 * If there is no overload, we just want to even the number of
>> -		 * idle cpus.
>> +		 * Allow for a small imbalance between NUMA groups; don't do any
>> +		 * of it if there is at least half as many tasks / busy CPUs as
>> +		 * there are available CPUs in the busiest group
>>  		 */
>> -		env->migration_type = migrate_task;
>> -		env->imbalance = max_t(long, 0, (local->idle_cpus -
>> -						 busiest->idle_cpus) >> 1);
>> +		if (env->sd->flags & SD_NUMA &&
>> +		    (busiest->sum_nr_running < busiest->group_weight >> 1) &&
> 
> This last line is not exactly equivalent to what I wrote. It would need
> to be
> 
> 	(busiest->sum_nr_running < (busiest->group_weight >> 1) - imbalance_adj) &&
> 

Right, I was implicitly suggesting that maybe we could forgo the
imbalance_adj computation and just roll with the imbalance_pct (with perhaps
and extra shift here and there). IMO the important thing here is the 
half-way cutoff.

> I can test as you suggest to see if it's roughly equivalent in terms of
> performance. The intent was to have a cutoff just before we reached 50%
> running tasks / busy CPUs.
> 

I think that cutoff makes sense; it's also important that it isn't purely
busy CPU-based because we're not guaranteed to have 1 task per CPU (due to
affinity or else), so I think the "half as many tasks as available CPUs"
thing has some merit.