linux-kernel - Re: [PATCH 1/2]percpu: introduce read mostly percpu API

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Message-ID: <1287561234.2328.1.camel@edumazet-laptop>
Date:	Wed, 20 Oct 2010 09:53:54 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Shaohua Li <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"hpa@...or.com" <hpa@...or.com>,
	"Chen, Tim C" <tim.c.chen@...el.com>
Subject: Re: [PATCH 1/2]percpu: introduce read mostly percpu API

Le mercredi 20 octobre 2010 à 09:35 +0200, Andi Kleen a écrit :
> On Wed, Oct 20, 2010 at 07:18:00AM +0200, Eric Dumazet wrote:
> > Le mercredi 20 octobre 2010 à 11:07 +0800, Shaohua Li a écrit :
> > > Add a new readmostly percpu section and api, next patch will use it.
> > > 
> > > Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> > > ---
> > 
> > 
> > Could you precisely describe why grouping together read mostly percpu
> > variables is a win ? Especially when you add in your next patch a single
> > variable ?
> 
> Not Shaohua, but I can explain it:
> 
> There is some per cpu data which is read by other CPUs. In this 
> case it's a win to use the separate section because you avoid false sharing 
> and the cache line can be kept in shared mode on all CPUs.
> 
> The next patch has an example of such data: data which is read
> by another CPU to send something to the target CPU.
> 
> I think the concept is useful and makes a lot of sense.
> The alternative would be __read_mostly NR_CPUS arrays, but 
> we all know that is a bad idea because it wastes too much memory
> on CONFIG_MAX_SMP setups.
> 

My question was more a rethoric one. I understand for sure the intent.

All this should be explained in changelog, so that people know
when/where use this new class of per_cpu variables ;)

Thanks


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