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Subject: Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks
To: Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>
References: <20160617120136.064100812@infradead.org>
 <20160617120454.150630859@infradead.org>
 <CAKfTPtAUtteP0V5-1u2n0YFDdFbZfwSqigHjOfM32Vbw3iAPbg@mail.gmail.com>
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 <20160617161831.GM30927@twins.programming.kicks-ass.net>
Cc: Yuyang Du <yuyang.du@intel.com>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Benjamin Segall <bsegall@google.com>, Paul Turner <pjt@google.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Matt Fleming <matt@codeblueprint.co.uk>
From: Dietmar Eggemann <dietmar.eggemann@arm.com>
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Date: Mon, 20 Jun 2016 12:35:59 +0100
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On 17/06/16 17:18, Peter Zijlstra wrote:
> On Fri, Jun 17, 2016 at 06:02:39PM +0200, Peter Zijlstra wrote:
>> So yes, ho-humm, how to go about doing that bestest. Lemme have a play.
> 
> This is what I came up with, not entirely pretty, but I suppose it'll
> have to do.
> 
> ---
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -724,6 +724,7 @@ void post_init_entity_util_avg(struct sc
>  	struct cfs_rq *cfs_rq = cfs_rq_of(se);
>  	struct sched_avg *sa = &se->avg;
>  	long cap = (long)(SCHED_CAPACITY_SCALE - cfs_rq->avg.util_avg) / 2;
> +	u64 now = cfs_rq_clock_task(cfs_rq);
>  
>  	if (cap > 0) {
>  		if (cfs_rq->avg.util_avg != 0) {
> @@ -738,7 +739,20 @@ void post_init_entity_util_avg(struct sc
>  		sa->util_sum = sa->util_avg * LOAD_AVG_MAX;
>  	}
>  
> -	update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, false);
> +	if (entity_is_task(se)) {
> +		struct task_struct *p = task_of(se);
> +		if (p->sched_class != &fair_sched_class) {
> +			/*
> +			 * For !fair tasks do attach_entity_load_avg()
> +			 * followed by detach_entity_load_avg() as per
> +			 * switched_from_fair().
> +			 */
> +			se->avg.last_update_time = now;
> +			return;
> +		}
> +	}
> +
> +	update_cfs_rq_load_avg(now, cfs_rq, false);
>  	attach_entity_load_avg(cfs_rq, se);
>  }
>  
> 

Doesn't a sleeping !fair_sched_class task which switches to fair uses
try_to_wake_up() rather than wake_up_new_task() so it won't go through
post_init_entity_util_avg()?