linux-kernel - Re: [PATCH v4 1/4] sched/fair: Introduce primitives for CFS bandwidth burst

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Message-ID: <YFCAXeZj6sXBI5Ls@hirez.programming.kicks-ass.net>
Date:   Tue, 16 Mar 2021 10:54:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Huaixin Chang <changhuaixin@...ux.alibaba.com>
Cc:     bsegall@...gle.com, dietmar.eggemann@....com,
        juri.lelli@...hat.com, khlebnikov@...dex-team.ru,
        linux-kernel@...r.kernel.org, mgorman@...e.de, mingo@...hat.com,
        odin@...d.al, odin@...dal.com, pauld@...head.com, pjt@...gle.com,
        rostedt@...dmis.org, shanpeic@...ux.alibaba.com, tj@...nel.org,
        vincent.guittot@...aro.org, xiyou.wangcong@...il.com
Subject: Re: [PATCH v4 1/4] sched/fair: Introduce primitives for CFS
 bandwidth burst

On Tue, Mar 16, 2021 at 12:49:28PM +0800, Huaixin Chang wrote:
> @@ -8982,6 +8983,12 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>  	if (quota != RUNTIME_INF && quota > max_cfs_runtime)
>  		return -EINVAL;
>  
> +	/*
> +	 * Bound burst to defend burst against overflow during bandwidth shift.
> +	 */
> +	if (burst > max_cfs_runtime)
> +		return -EINVAL;

Why do you allow such a large burst? I would expect something like:

	if (burst > quote)
		return -EINVAL;

That limits the variance in the system. Allowing super long bursts seems
to defeat the entire purpose of bandwidth control.