linux-kernel - Re: [PATCH v3 1/4] sched/fair: Introduce primitives for CFS bandwidth burst

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Date:   Wed, 10 Mar 2021 13:39:49 +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,
        pauld@...head.com, pjt@...gle.com, rostedt@...dmis.org,
        shanpeic@...ux.alibaba.com, vincent.guittot@...aro.org,
        xiyou.wangcong@...il.com
Subject: Re: [PATCH v3 1/4] sched/fair: Introduce primitives for CFS
 bandwidth burst

On Thu, Jan 21, 2021 at 07:04:50PM +0800, Huaixin Chang wrote:
> In this patch, we introduce the notion of CFS bandwidth burst. Unused
> "quota" from pervious "periods" might be accumulated and used in the
> following "periods". The maximum amount of accumulated bandwidth is
> bounded by "burst". And the maximun amount of CPU a group can consume in
> a given period is "buffer" which is equivalent to "quota" + "burst in
> case that this group has done enough accumulation.
> 
> Signed-off-by: Huaixin Chang <changhuaixin@...ux.alibaba.com>
> Signed-off-by: Shanpei Chen <shanpeic@...ux.alibaba.com>

Not a valid SoB chain.

> ---
>  kernel/sched/core.c  | 91 ++++++++++++++++++++++++++++++++++++++++++++--------
>  kernel/sched/fair.c  |  2 ++
>  kernel/sched/sched.h |  2 ++
>  3 files changed, 82 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e7e453492cff..48d3bad12be2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7895,10 +7895,12 @@ static const u64 max_cfs_runtime = MAX_BW * NSEC_PER_USEC;
>  
>  static int __cfs_schedulable(struct task_group *tg, u64 period, u64 runtime);
>  
> -static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
> +static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota,
> +							u64 burst)

Non standard indentation.

>  {
>  	int i, ret = 0, runtime_enabled, runtime_was_enabled;
>  	struct cfs_bandwidth *cfs_b = &tg->cfs_bandwidth;
> +	u64 buffer;
>  
>  	if (tg == &root_task_group)
>  		return -EINVAL;
> @@ -7925,6 +7927,16 @@ 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;
> +
> +	if (quota == RUNTIME_INF)
> +		buffer = RUNTIME_INF;
> +	else
> +		buffer = min(max_cfs_runtime, quota + burst);

Why do we care about buffer when RUNTIME_INF?