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

From: Raghavendra K T
Date: Sat Aug 29 2015 - 13:25:16 EST

On 08/29/2015 08:51 PM, Joe Perches wrote:
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

Yes, as also suggested by Eric.

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);

Sure. It makes it more readable.
will respin V4 with these changes (+ memset 0 for pad which I realized).

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