linux-kernel - Re: [PATCH RFC V3 2/2] net: Optimize snmp stat aggregation by walking all the percpu data at once

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Message-ID: <1440858726.8932.149.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sat, 29 Aug 2015 07:32:06 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	davem@...emloft.net, kuznet@....inr.ac.ru, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net, jiri@...nulli.us,
	edumazet@...gle.com, hannes@...essinduktion.org,
	tom@...bertland.com, azhou@...ira.com, ebiederm@...ssion.com,
	ipm@...rality.org.uk, nicolas.dichtel@...nd.com,
	serge.hallyn@...onical.com, joe@...ches.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	anton@....ibm.com, nacc@...ux.vnet.ibm.com,
	srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH RFC V3 2/2] net: Optimize snmp stat aggregation by
 walking all the percpu data at once

On Sat, 2015-08-29 at 14:37 +0530, Raghavendra K T wrote:

>  
>  static inline void __snmp6_fill_stats64(u64 *stats, void __percpu *mib,
> -				      int items, int bytes, size_t syncpoff)
> +					int items, int bytes, size_t syncpoff)
>  {
> -	int i;
> +	int i, c;
>  	int pad = bytes - sizeof(u64) * items;
> +	u64 buff[items];
> +

One last comment : using variable length arrays is confusing for the
reader, and sparse as well.

$ make C=2 net/ipv6/addrconf.o
...
  CHECK   net/ipv6/addrconf.c
net/ipv6/addrconf.c:4733:18: warning: Variable length array is used.
net/ipv6/addrconf.c:4737:25: error: cannot size expression


I suggest you remove 'items' parameter and replace it by
IPSTATS_MIB_MAX, as __snmp6_fill_stats64() is called exactly once.



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