netdev - Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by walking all the percpu data at once

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Date:	Fri, 28 Aug 2015 15:29:40 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	David Miller <davem@...emloft.net>,
	raghavendra.kt@...ux.vnet.ibm.com, edumazet@...gle.com,
	kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
	kaber@...sh.net, jiri@...nulli.us, 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 V2 2/2] net: Optimize snmp stat aggregation by
 walking all the percpu data at once

On Fri, 2015-08-28 at 14:26 -0700, Joe Perches wrote:

> Always a possibility, but I don't think so.
> 
> > put_unaligned is happening on a space allocated from rtnetlink skb, not
> > the temp space needed to perform the per cpu folding.
> 
> That's why I suggested changing the snmp_fill_stats arguments.
> 
> If the naturally aligned allocated u64 array is used and then
> copied as a block to the rtnetlink skb, I believe there's no
> alignment issue that would require put_unaligned.

1) u64 array[XX] on stack is naturally aligned,
kzalloc() wont improve this at all. Not sure what you believe.

2) put_unaligned() is basically a normal memory write on x86.
 memcpy(dst,src,...) will have a problem anyway on arches that care,
because src & dst wont have same alignment.

288 bytes on stack in a leaf function in this path is totally fine, it
is not like we're calling ext4/xfs/nfs code after this point.


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