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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Date:	Sat, 29 Aug 2015 08:21:24 -0700
From:	Joe Perches <joe@...ches.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	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, 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 07:32 -0700, Eric Dumazet wrote:
> 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.

If you respin, I suggest:

o remove "items" from the __snmp6_fill_stats64 arguments
  and use IPSTATS_MIB_MAX in the function instead

o add braces around the for_each_possible_cpu loop

	for_each_possible_cpu(c) {
		for (i = 1; i < items; i++)
			buff[i] += snmp_get_cpu_field64(mib, c, i, syncpoff);
	}


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