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

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Message-ID: <1440808524.11525.189.camel@perches.com>
Date:	Fri, 28 Aug 2015 17:35:24 -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 17:06 -0700, Eric Dumazet wrote:
> On Fri, 2015-08-28 at 16:12 -0700, Joe Perches wrote: 
> > Generally true.  It's always difficult to know how much
> > stack has been consumed though and smaller stack frames
> > are generally better.
[] 
> So for a _leaf_ function, it is better to declare an automatic variable,
> as you in fact reduce max stack depth.

That of course depends on what a "leaf" is and
whether or not any other function call in the
"leaf" consumes stack.

inet6_fill_ifla6_attrs does call other functions
(none of which has the stack frame size of k.alloc)

> Not only it uses less kernel stack, it is also way faster, as you avoid
> kmalloc()/kfree() overhead and reuse probably already hot cache lines in
> kernel stack.

yup.

You'll also never neglect to free stack like the
original RFC patch neglected to free the alloc.

cheers, Joe

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