Re: [PATCH] sched/cfs: fix spurious active migration

[Peter Zijlstra]

On Fri, Nov 29, 2019 at 03:04:47PM +0100, Vincent Guittot wrote:
> The load balance can fail to find a suitable task during the periodic check
> because  the imbalance is smaller than half of the load of the waiting
> tasks. This results in the increase of the number of failed load balance,
> which can end up to start an active migration. This active migration is
> useless because the current running task is not a better choice than the
> waiting ones. In fact, the current task was probably not running but
> waiting for the CPU during one of the previous attempts and it had already
> not been selected.
> 
> When load balance fails too many times to migrate a task, we should relax
> the contraint on the maximum load of the tasks that can be migrated
> similarly to what is done with cache hotness.
> 
> Before the rework, load balance used to set the imbalance to the average
> load_per_task in order to mitigate such situation. This increased the
> likelihood of migrating a task but also of selecting a larger task than
> needed while more appropriate ones were in the list.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
> 
> I haven't seen any noticable performance changes on the benchmarks that I
> usually run but the problem can be easily highlight with a simple test
> with 9 always running tasks on 8 cores.
> 
>  kernel/sched/fair.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e0d662a..d1b4fa7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7433,7 +7433,14 @@ static int detach_tasks(struct lb_env *env)
>  			    load < 16 && !env->sd->nr_balance_failed)
>  				goto next;
>  
> -			if (load/2 > env->imbalance)
> +			/*
> +			 * Make sure that we don't migrate too much load.
> +			 * Nevertheless, let relax the constraint if
> +			 * scheduler fails to find a good waiting task to
> +			 * migrate.
> +			 */
> +			if (load/2 > env->imbalance &&
> +			    env->sd->nr_balance_failed <= env->sd->cache_nice_tries)
>  				goto next;
>  
>  			env->imbalance -= load;

The alternative is carrying a flag that inhibits incrementing
nr_balance_failed.

Not migrating anything when doing so would make the imbalance worse is
not a failure after all.