Messages in this thread Patch in this message | | | From | Vincent Guittot <> | Subject | [PATCH] sched/cfs: fix spurious active migration | Date | Fri, 29 Nov 2019 15:04:47 +0100 |
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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@linaro.org> ---
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; -- 2.7.4
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