linux-kernel - Re: [RFC 02/18] cgroup_pids: track maximum pids

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Message-ID: <20160613211227.GG31708@htj.duckdns.org>
Date:	Mon, 13 Jun 2016 17:12:27 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Topi Miettinen <toiwoton@...il.com>
Cc:	linux-kernel@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>
Subject: Re: [RFC 02/18] cgroup_pids: track maximum pids

Hello,

On Mon, Jun 13, 2016 at 10:44:09PM +0300, Topi Miettinen wrote:
> Track maximum pids in the cgroup, present it in cgroup pids.current_max.

"max" is often used for maximum limits in cgroup.  I think "watermark"
or "high_watermark" would be a lot clearer.

> @@ -236,6 +246,14 @@ static void pids_free(struct task_struct *task)
>  	pids_uncharge(pids, 1);
>  }
>  
> +static void pids_fork(struct task_struct *task)
> +{
> +	struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
> +
> +	if (atomic64_read(&pids->cur_max) < atomic64_read(&pids->counter))
> +		atomic64_set(&pids->cur_max, atomic64_read(&pids->counter));
> +}

Wouldn't it make more sense to track high watermark from the charge
functions instead?  I don't get why this requires a separate fork
callback.  Also, racing atomic64_set's are racy.  The counter can end
up with a lower number than it should be.

> @@ -300,6 +326,11 @@ static struct cftype pids_files[] = {
>  		.read_s64 = pids_current_read,
>  		.flags = CFTYPE_NOT_ON_ROOT,
>  	},
> +	{
> +		.name = "current_max",

Please make this "high_watermark" field in pids.stats file.

Thanks.

-- 
tejun