Re: [PATCH 1/8] sched/fair: Clean up active balance nr_balance_failed trickery

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

>> @@ -9805,9 +9810,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>  					active_load_balance_cpu_stop, busiest,
>>  					&busiest->active_balance_work);
>>  			}
>> -
>> -			/* We've kicked active balancing, force task migration. */
>> -			sd->nr_balance_failed = sd->cache_nice_tries+1;
>
> This has an impact on future calls to need_active_balance() too, no? We enter
> this path because need_active_balance() returned true; one of the conditions it
> checks for is
>
> 	return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
>
> So since we used to reset nr_balanced_failed to cache_nice_tries+1, the above
> condition would be false in the next call or two IIUC. But since we remove
> that, we could end up here again soon.
>
> Was this intentional?
>

Partially, I'd say :-)

If you look at active_load_balance_cpu_stop(), it does

  sd->nr_balance_failed = 0;

when it successfully pulls a task. So we get a reset of the failed counter
on pull, which I've preserved. As for interactions with later
need_active_balance(), the commit that introduced the current counter write
(which is over 15 years old!):  

  3950745131e2 ("[PATCH] sched: fix SMT scheduling problems")

only states the task_hot() issue; thus I'm doubtful whether said
interaction was intentional.