linux-kernel - Re: [PATCH 2/2] sched/core: Avoid unnecessary update in tg_set_cfs

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Message-ID: <c60c07b3-7c97-4c9d-8496-72f3fc65307c@linux.dev>
Date: Mon, 22 Jul 2024 15:21:40 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Chuyi Zhou <zhouchuyi@...edance.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com
Cc: linux-kernel@...r.kernel.org, joshdon@...gle.com
Subject: Re: [PATCH 2/2] sched/core: Avoid unnecessary update in
 tg_set_cfs_bandwidth

On 2024/7/21 20:52, Chuyi Zhou wrote:
> In the kubernetes production environment, we have observed a high
> frequency of writes to cpu.max, approximately every 2~4 seconds for each
> cgroup, with the same value being written each time. This can result in
> unnecessary overhead, especially on machines with a large number of CPUs
> and cgroups.
> 
> This is because kubelet and runc attempt to persist resource
> configurations through frequent updates with same value in this manner.

Ok.

> While optimizations can be made to kubelet and runc to avoid such
> overhead(e.g. check the current value of cpu request/limit before writing
> to cpu.max), it is still worth to bail out from tg_set_cfs_bandwidth() if
> we attempt to update with the same value.

Yeah, we can optimize this situation with a little of checking code,
seems worthwhile to do IMHO.

> 
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
> ---
>   kernel/sched/core.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 6d35c48239be..4db3ef2a703b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9081,6 +9081,8 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota,
>   				     burst + quota > max_cfs_runtime))
>   		return -EINVAL;
>   
> +	if (cfs_b->period == ns_to_ktime(period) && cfs_b->quota == quota && cfs_b->burst == burst)
> +		return 0;

Maybe we'd better do these checkings under the lock protection, right?

Thanks.

>   	/*
>   	 * Prevent race between setting of cfs_rq->runtime_enabled and
>   	 * unthrottle_offline_cfs_rqs().