linux-kernel - 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 14:26:12 -0700
From:	Joe Perches <joe@...ches.com>
To:	Eric Dumazet <eric.dumazet@...il.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 14:14 -0700, Eric Dumazet wrote:
> On Fri, 2015-08-28 at 14:09 -0700, Joe Perches wrote:
> > On Fri, 2015-08-28 at 13:55 -0700, Eric Dumazet wrote:
> > > On Fri, 2015-08-28 at 13:53 -0700, Joe Perches wrote:
> > > > It's 288 bytes on stack, maybe a kzalloc would be clearer too.
> > > 
> > > Could you read patch history and check why this has been rejected ?
> > 
> > I don't see a rejection, just that the initial
> > submission didn't check the allocation or add
> > an allocation buffer via kcalloc/kzalloc to the
> > inet6_fill_ifla6_attrs caller and change the
> > snmp6_fill_stats arguments.
> > 
> > It could also eliminate the put_unaligned calls.
> 
> Not really. You do not properly read this code.

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.

Do I still miss something?

> https://lkml.org/lkml/2015/8/25/476

I read that.

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