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: <55E1EB14.5040205@linux.vnet.ibm.com>
Date:	Sat, 29 Aug 2015 22:55:40 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Joe Perches <joe@...ches.com>
CC:	Eric Dumazet <eric.dumazet@...il.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 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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