Messages in this thread | ![/](/images/icornerl.gif) | | From | Vincent Guittot <> | Date | Thu, 13 Dec 2018 12:04:20 +0100 | Subject | Re: [PATCH v2] sched/fair: fix 1 task per CPU |
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On Thu, 13 Dec 2018 at 11:44, Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Sep 10, 2018 at 04:43:09PM +0200, Vincent Guittot wrote: > > When CPUs have different capacity because of RT/DL tasks or > > micro-architecture or max frequency differences, there are situation where > > the imbalance is not correctly set to migrate waiting task on the idle CPU. > > > > The UC uses the force_balance case: > > > > if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) && > > busiest->group_no_capacity) > > goto force_balance; > > > > But calculate_imbalance fails to set the right amount of load to migrate > > a task because of the special condition: > > > > busiest->avg_load <= sds->avg_load || local->avg_load >= sds->avg_load) > > > > Add in fix_small_imbalance, this special case that triggered the force > > balance in order to make sure that the amount of load to migrate will be > > enough. > > So I think this patch is going in the wrong direction for a number of > reasons: > > - we'd like to get rid of fix_small_imbalance(), and this adds to it; > > - the whole load_per_task stuff is terminally broken, it _cannot_ work > right. > > > What I've suggested in the past is parameterizing the load balancer and > picking different criteria to balance on:
This patch is clearly a fix of the current implementation. What you suggest below makes sense but implies a significant rework in the calculate_imbalance and the load_balancer in general and will need more time to reach a stable state. Nevertheless, I will have a look at that
I imagine that your feedback for https://lkml.org/lkml/2018/10/2/283 will be the same ?
Vincent
> > - nr_running ; if there are idle CPUs around > - utilization ; if there's idle time (u<1) > - weight > > And I suppose you're hitting one of the nr_running cases here. > > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > --- > > kernel/sched/fair.c | 14 ++++++++++++++ > > 1 file changed, 14 insertions(+) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 309c93f..72bc5e8 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -8048,6 +8048,20 @@ void fix_small_imbalance(struct lb_env *env, struct sd_lb_stats *sds) > > local = &sds->local_stat; > > busiest = &sds->busiest_stat; > > > > + /* > > + * There is available capacity in local group and busiest group is > > + * overloaded but calculate_imbalance can't compute the amount of load > > + * to migrate because load_avg became meaningless due to asymetric > > + * capacity between groups. In such case, we only want to migrate at > > + * least one tasks of the busiest group and rely of the average load > > + * per task to ensure the migration. > > + */ > > + if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) && > > + busiest->group_no_capacity) { > > + env->imbalance = busiest->load_per_task; > > + return; > > + } > > + > > if (!local->sum_nr_running) > > local->load_per_task = cpu_avg_load_per_task(env->dst_cpu); > > else if (busiest->load_per_task > local->load_per_task) > > -- > > 2.7.4 > >
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