linux-kernel - Re: [PATCH] sched/pelt: fix update_blocked

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Message-ID: <20180831144146.GO24124@hirez.programming.kicks-ass.net>
Date:   Fri, 31 Aug 2018 16:41:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/pelt: fix update_blocked_averages() for dl and rt

On Fri, Aug 31, 2018 at 03:58:28PM +0200, Vincent Guittot wrote:
> update_blocked_averages() is called to periodiccally decay the stalled load
> of idle CPUs and to sync all loads before running load balance.
> 
> When cfs rq is idle, it trigs a load balance during pick_next_task_fair()
> in order to potentially pull tasks and to use this newly idle CPU. This
> load balance happens whereas prev task from another class has not been put
> and its utilization updated yet. This may lead to wrongly account running
> time as idle time for rt or dl classes.
> 
> Test that no rt or dl task is running when updating their utilization in
> update_blocked_averages().
> 
> We still update rt and dl utilization instead of simply skipping them to
> make sure that all metrics are synced when used during load balance.
> 

Fixes: 371bf4273269 ("sched/rt: Add rt_rq utilization tracking")
Fixes: 3727e0e16340 ("sched/dl: Add dl_rq utilization tracking")

Right?

> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 309c93f..a1babaf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7262,6 +7262,7 @@ static void update_blocked_averages(int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);
>  	struct cfs_rq *cfs_rq, *pos;
> +	const struct sched_class *curr_class = rq->curr->sched_class;
>  	struct rq_flags rf;
>  	bool done = true;
>  
> @@ -7298,8 +7299,8 @@ static void update_blocked_averages(int cpu)
>  		if (cfs_rq_has_blocked(cfs_rq))
>  			done = false;
>  	}
> -	update_rt_rq_load_avg(rq_clock_task(rq), rq, 0);
> -	update_dl_rq_load_avg(rq_clock_task(rq), rq, 0);
> +	update_rt_rq_load_avg(rq_clock_task(rq), rq, curr_class == &rt_sched_class);
> +	update_dl_rq_load_avg(rq_clock_task(rq), rq, curr_class == &dl_sched_class);
>  	update_irq_load_avg(rq, 0);
>  	/* Don't need periodic decay once load/util_avg are null */
>  	if (others_have_blocked(rq))

Did you forget to update the second implementation of
update_blocked_averages() ?