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

From: Joe Perches <[email-masked]>
Date: Sat, 29 Aug 2015 08:21:24 -0700

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);
	}