linux-kernel - Re: [RFC V2] proc: change /proc/stat show

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Message-Id: <20150318150734.f725e75a307601c6214e5b73@linux-foundation.org>
Date:	Wed, 18 Mar 2015 15:07:34 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>
Cc:	"'David Rientjes'" <rientjes@...gle.com>,
	"'heiko.carstens@...ibm.com'" <heiko.carstens@...ibm.com>,
	"'tglx@...utronix.de'" <tglx@...utronix.de>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC V2] proc: change /proc/stat show

On Wed, 18 Mar 2015 10:35:39 +0800 "Wang, Yalin" <Yalin.Wang@...ymobile.com> wrote:

> This patch change /proc/stat to show each cpu,
> we show each present cpus instead of eacn online cpu,
> because some cpus are online / offline dynamically,
> we should also show its cputime even it is offline,
> some lib will read this file to detect cpu numbers,
> we should also return the real present cpu numbers,
> not just online cpus.

/proc/cpuinfo also skips offline CPUs.

> --- a/fs/proc/stat.c
> +++ b/fs/proc/stat.c
> @@ -130,7 +130,7 @@ static int show_stat(struct seq_file *p, void *v)
>  	seq_put_decimal_ull(p, ' ', cputime64_to_clock_t(guest_nice));
>  	seq_putc(p, '\n');
>  
> -	for_each_online_cpu(i) {
> +	for_each_present_cpu(i) {
>  		/* Copy values here to work around gcc-2.95.3, gcc-2.96 */
>  		user = kcpustat_cpu(i).cpustat[CPUTIME_USER];
>  		nice = kcpustat_cpu(i).cpustat[CPUTIME_NICE];

It's a non-back-compatible userspace interface change and I don't know
what might break as a result.  For example, anyone who is using
/proc/stat to find out which CPUs are online will get a surprise!


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