linux-kernel - Re: [RFC PATCH 2/3] net: Replace for_each_possible_cpu with for_each_online

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Message-ID: <1470737431.5324.0.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Tue, 09 Aug 2016 12:10:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jia He <hejianet@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [RFC PATCH 2/3] net: Replace for_each_possible_cpu with
 for_each_online_cpu

On Mon, 2016-08-08 at 18:22 +0800, Jia He wrote:
> In PowerPC server with large number cpus, the loop index in smt=1 could be 
> reduced to 1/8 compared with smt=8.
> Thus cache misses can be reduced.
> 
> Signed-off-by: Jia He <hejianet@...il.com>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Alexey Kuznetsov <kuznet@....inr.ac.ru>
> Cc: James Morris <jmorris@...ei.org>
> Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
> Cc: Patrick McHardy <kaber@...sh.net>
> ---
>  net/ipv6/addrconf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index 43fa8d0..1fce613 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -4969,7 +4969,7 @@ static inline void __snmp6_fill_stats64(u64 *stats, void __percpu *mib,
>  
>  	buff[0] = IPSTATS_MIB_MAX;
>  
> -	for_each_possible_cpu(c) {
> +	for_each_online_cpu(c) {
>  		for (i = 1; i < IPSTATS_MIB_MAX; i++)
>  			buff[i] += snmp_get_cpu_field64(mib, c, i, syncpoff);
>  	}

This will break on machines with cpu hotplug.