linux-kernel - Re: [PATCH v2] sched/fair: Remove setting task's se->runnable

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Message-ID: <bf1ed7b3-e67f-fbf0-a32f-ad5d7dfa5731@arm.com>
Date:   Mon, 10 Sep 2018 16:49:05 -0700
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Quentin Perret <quentin.perret@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: Remove setting task's se->runnable_weight
 during PELT update

Hi,

On 08/03/2018 07:05 AM, Dietmar Eggemann wrote:
> A CFS (SCHED_OTHER, SCHED_BATCH or SCHED_IDLE policy) task's
> se->runnable_weight must always be in sync with its se->load.weight.
> 
> se->runnable_weight is set to se->load.weight when the task is
> forked (init_entity_runnable_average()) or reniced (reweight_entity()).
> 
> There are two cases in set_load_weight() which since they currently only
> set se->load.weight could lead to a situation in which se->load.weight
> is different to se->runnable_weight for a CFS task:
> 
> (1) A task switches to SCHED_IDLE.
> 
> (2) A SCHED_FIFO, SCHED_RR or SCHED_DEADLINE task which has been reniced
>      (during which only its static priority gets set) switches to
>      SCHED_OTHER or SCHED_BATCH.
> 
> Set se->runnable_weight to se->load.weight in these two cases to prevent
> this. This eliminates the need to explicitly set it to se->load.weight
> during PELT updates in the CFS scheduler fastpath.
> 
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> ---
> 
> Changes v1->v2:
> - Rebased on latest tip/sched/core
> 
> This patch has been tested w/ appropriate BUG_ON()'s in
> __update_load_avg_blocked_se() and __update_load_avg_se() on an Ubuntu
> 18.04 desktop.
> 
>   kernel/sched/core.c | 2 ++
>   kernel/sched/pelt.c | 6 ------
>   2 files changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index deafa9fe602b..2a08418db3d1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -701,6 +701,7 @@ static void set_load_weight(struct task_struct *p, bool update_load)
>   	if (idle_policy(p->policy)) {
>   		load->weight = scale_load(WEIGHT_IDLEPRIO);
>   		load->inv_weight = WMULT_IDLEPRIO;
> +		p->se.runnable_weight = load->weight;
>   		return;
>   	}
>   
> @@ -713,6 +714,7 @@ static void set_load_weight(struct task_struct *p, bool update_load)
>   	} else {
>   		load->weight = scale_load(sched_prio_to_weight[prio]);
>   		load->inv_weight = sched_prio_to_wmult[prio];
> +		p->se.runnable_weight = load->weight;
>   	}
>   }
>   
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index 35475c0c5419..d0016b16d23a 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -269,9 +269,6 @@ ___update_load_avg(struct sched_avg *sa, unsigned long load, unsigned long runna
>   
>   int __update_load_avg_blocked_se(u64 now, int cpu, struct sched_entity *se)
>   {
> -	if (entity_is_task(se))
> -		se->runnable_weight = se->load.weight;
> -
>   	if (___update_load_sum(now, cpu, &se->avg, 0, 0, 0)) {
>   		___update_load_avg(&se->avg, se_weight(se), se_runnable(se));
>   		return 1;
> @@ -282,9 +279,6 @@ int __update_load_avg_blocked_se(u64 now, int cpu, struct sched_entity *se)
>   
>   int __update_load_avg_se(u64 now, int cpu, struct cfs_rq *cfs_rq, struct sched_entity *se)
>   {
> -	if (entity_is_task(se))
> -		se->runnable_weight = se->load.weight;
> -
>   	if (___update_load_sum(now, cpu, &se->avg, !!se->on_rq, !!se->on_rq,
>   				cfs_rq->curr == se)) {
>   
> 

Is there anything else I should do for this patch ?

Thanks,

-- Dietmar