linux-kernel - Re: [PATCH] proc: only export statistics of softirqs for online cpus

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Message-ID: <20190111230343.GA20267@avx2>
Date:   Sat, 12 Jan 2019 02:03:43 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Tan Hu <tan.hu@....com.cn>
Cc:     zhong.weidong@....com.cn, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] proc: only export statistics of softirqs for online cpus

On Thu, Jan 10, 2019 at 07:24:01PM +0800, Tan Hu wrote:
> @@ -12,13 +12,13 @@ static int show_softirqs(struct seq_file *p, void *v)
>  	int i, j;
>  
>  	seq_puts(p, "                    ");
> -	for_each_possible_cpu(i)
> +	for_each_online_cpu(i)
>  		seq_printf(p, "CPU%-8d", i);
>  	seq_putc(p, '\n');
>  
>  	for (i = 0; i < NR_SOFTIRQS; i++) {
>  		seq_printf(p, "%12s:", softirq_to_name[i]);
> -		for_each_possible_cpu(j)
> +		for_each_online_cpu(j)
>  			seq_printf(p, " %10u", kstat_softirqs_cpu(i, j));
>  		seq_putc(p, '\n');
>  	}

This should break userspace:

	https://sources.debian.org/src/netsniff-ng/0.6.5-1/ifpps.c/#L330

This code gets the number of "possible" CPUs from sysconf(3) and doesn't
parse header to find out which CPUs are online.