Messages in this thread | | | Date | Tue, 12 Jul 2022 11:52:50 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH v2] sched/fair: fix case with reduced capacity CPU |
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On 07/11/22 18:42, Vincent Guittot wrote: > On Mon, 11 Jul 2022 at 18:03, Qais Yousef <qais.yousef@arm.com> wrote: > > > > Hi Vincent > > > > On 07/08/22 17:44, Vincent Guittot wrote: > > > The capacity of the CPU available for CFS tasks can be reduced because of > > > other activities running on the latter. In such case, it's worth trying to > > > move CFS tasks on a CPU with more available capacity. > > > > > > The rework of the load balance has filtered the case when the CPU is > > > classified to be fully busy but its capacity is reduced. > > > > > > Check if CPU's capacity is reduced while gathering load balance statistic > > > and classify it group_misfit_task instead of group_fully_busy so we can > > > try to move the load on another CPU. > > > > > > Reported-by: David Chen <david.chen@nutanix.com> > > > Reported-by: Zhang Qiao <zhangqiao22@huawei.com> > > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > > Tested-by: David Chen <david.chen@nutanix.com> > > > Tested-by: Zhang Qiao <zhangqiao22@huawei.com> > > > --- > > > > [...] > > > > > @@ -8820,8 +8833,9 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > > > > > for_each_cpu_and(i, sched_group_span(group), env->cpus) { > > > struct rq *rq = cpu_rq(i); > > > + unsigned long load = cpu_load(rq); > > > > > > - sgs->group_load += cpu_load(rq); > > > + sgs->group_load += load; > > > sgs->group_util += cpu_util_cfs(i); > > > sgs->group_runnable += cpu_runnable(rq); > > > sgs->sum_h_nr_running += rq->cfs.h_nr_running; > > > @@ -8851,11 +8865,17 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > > if (local_group) > > > continue; > > > > > > - /* Check for a misfit task on the cpu */ > > > - if (env->sd->flags & SD_ASYM_CPUCAPACITY && > > > - sgs->group_misfit_task_load < rq->misfit_task_load) { > > > - sgs->group_misfit_task_load = rq->misfit_task_load; > > > - *sg_status |= SG_OVERLOAD; > > > + if (env->sd->flags & SD_ASYM_CPUCAPACITY) { > > > + /* Check for a misfit task on the cpu */ > > > + if (sgs->group_misfit_task_load < rq->misfit_task_load) { > > > + sgs->group_misfit_task_load = rq->misfit_task_load; > > > + *sg_status |= SG_OVERLOAD; > > > + } > > > + } else if ((env->idle != CPU_NOT_IDLE) && > > > + sched_reduced_capacity(rq, env->sd)) { > > > + /* Check for a task running on a CPU with reduced capacity */ > > > + if (sgs->group_misfit_task_load < load) > > > + sgs->group_misfit_task_load = load; > > > } > > > } > > > > Small questions mostly for my education purposes. > > > > The new condition only applies for SMP systems. The reason asym systems don't > > care is because misfit check already considers capacity pressure when checking > > that the task fits_capacity()? > > Yes > > > > > It **seems** to me that the migration margin in fits_capacity() acts like the > > sd->imbalance_pct when check_cpu_capacity() is called by > > sched_reduced_capacity(), did I get it right? > > Yes > > > > > If I got it right, if the migration margin ever tweaked, could we potentially > > start seeing this kind of reported issue on asym systems then? I guess not. It > > just seems to me for asym systems tweaking the migration margin is similar to > > tweaking imbalance_pct for smp ones. But the subtlety is greater as > > imbalance_pct is still used in asym systems. > > You should not because the task will end up being misfit whatever the > margin. The only change would be how fast you will detect and migrate
Yes. It's just this control/knob of how fast is different for HMP vs SMP.
check_misfit_status() does rely on rq->misfit_task_load AND check_cpu_capacity(). So for that case the 2 knobs will impact how fast this check will converge.
I want to say we should document this with a comment, but tbh I have no clue how this can be explained clearly without being way too verbose.
All looks good to me anyway. Thanks for clarifying.
Cheers
-- Qais Yousef
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