Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 14 Sep 2018 18:26:59 +0200 | Subject | Re: [PATCH v2] sched/fair: fix 1 task per CPU |
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On Fri, 14 Sep 2018 at 05:22, Valentin Schneider <valentin.schneider@arm.com> wrote: > > Hi, > > On 10/09/18 07:43, 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. > > > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > Other than the commit nit, LGTM. Out of curiosity I ran some kernel compile > on my HiKey960 (-j8) but didn't see much change - something along the lines > of ~1% speedup, and although it was consistent over a few iterations, I'd > need a whole lot more of them to back this up. > > I kind of expected it because some sporadic task can show up and tip the > scale in the right direction, so even without the patch the situation can > "fix itself" eventually, and it becomes less noticeable on really long > workloads.
I have seen a better stdev and shorter duration for the tests that you used for misfit patch. The test have been done with asym packing and the few fixes that I sent in another patchset for asym packing
> > I do see a difference by looking at the trace of a simple 8 100% tasks rt-app > workload though, as I no longer see that idling LITTLE I sometimes get > without the patch, which is what we expect, so: > > Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Thanks
> > > --- > > Again, I'd argue for a slightly more explicit header. As you pointed out in > v1, it's not just long running tasks, so maybe just "fix 1 *running* task per > CPU"? Otherwise I feel it's a tad obscure.
To be honest i don't mind about header but I don't see the benefit of adding *running*. So I let Peter or Ingo decide what they prefer
> > > 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. > > Could you add something along the lines of "(see similar condition in > find_busiest_group())"? > > 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) > >
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