linux-kernel - Re: [PATCH 5/7 v4] sched: propagate asynchrous detach

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Date:   Wed, 12 Oct 2016 16:03:20 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org, kernellwp@...il.com
Cc:     yuyang.du@...el.com, Morten.Rasmussen@....com,
        linaro-kernel@...ts.linaro.org, pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH 5/7 v4] sched: propagate asynchrous detach

On 26/09/16 13:19, Vincent Guittot wrote:
> A task can be asynchronously detached from cfs_rq when migrating
> between CPUs. The load of the migrated task is then removed from
> source cfs_rq during its next update. We use this event to set propagation
> flag.
> 
> During the load balance, we take advanatge of the update of blocked load
> to we propagate any pending changes.

IMHO, it would be a good idea to mention that '2/7 sched: fix
hierarchical order in rq->leaf_cfs_rq_list' is a hard requirement for
this to work. The functionality relies on the order of cfs_rq's (top to
root) in the rq->leaf_cfs_rq_list list.

> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8ba500f..bd3b6b9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3221,6 +3221,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
>  		sub_positive(&sa->load_avg, r);
>  		sub_positive(&sa->load_sum, r * LOAD_AVG_MAX);
>  		removed_load = 1;
> +		set_tg_cfs_propagate(cfs_rq);
>  	}
>  
>  	if (atomic_long_read(&cfs_rq->removed_util_avg)) {
> @@ -3228,6 +3229,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
>  		sub_positive(&sa->util_avg, r);
>  		sub_positive(&sa->util_sum, r * LOAD_AVG_MAX);
>  		removed_util = 1;
> +		set_tg_cfs_propagate(cfs_rq);
>  	}
>  
>  	decayed = __update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
> @@ -6607,6 +6609,10 @@ static void update_blocked_averages(int cpu)
>  
>  		if (update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, true))
>  			update_tg_load_avg(cfs_rq, 0);
> +
> +		/* Propagate pending load changes to the parent */
> +		if (cfs_rq->tg->se[cpu])
> +			update_load_avg(cfs_rq->tg->se[cpu], 0);

In my test (1 task (run/period: 8ms/16ms) in tg_root->tg_x->tg_y->*tg_z*
and oscillating between cpu1 and cpu2) the cfs_rq related signals are
nicely going down to 0 after the task has left the cpu but it doesn't
seem to be the case for the corresponding se (cfs_rq->tg->se[cpu])?

It should actually work correctly because of the
update_tg_cfs_util/load() calls in update_load_avg(cfs_rq->tg->se[cpu],
0)->propagate_entity_load_avg()

[...]