linux-kernel - Re: [RFC PATCH] tg: count the sum wait time of an task group

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Message-ID: <20180702120356.GJ2494@hirez.programming.kicks-ass.net>
Date:   Mon, 2 Jul 2018 14:03:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] tg: count the sum wait time of an task group

On Mon, Jul 02, 2018 at 03:29:39PM +0800, 王贇 wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1866e64..ef82ceb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -862,6 +862,7 @@ static void update_curr_fair(struct rq *rq)
>  static inline void
>  update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> +	struct task_group *tg;
>  	struct task_struct *p;
>  	u64 delta;
> 
> @@ -882,6 +883,9 @@ static void update_curr_fair(struct rq *rq)
>  			return;
>  		}
>  		trace_sched_stat_wait(p, delta);
> +	} else {
> +		tg = group_cfs_rq(se)->tg;
> +		__schedstat_add(tg->wait_sum, delta);
>  	}

You're joking right? This patch is both broken and utterly insane.

You're wanting to update an effectively global variable for every
schedule action (and its broken because it is without any serialization
or atomics).

NAK