netdev - Re: [PATCH] percpu: add optimized generic percpu accessors

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Date:	Tue, 20 Jan 2009 11:36:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org, hpa@...or.com, brgerst@...il.com,
	ebiederm@...ssion.com, cl@...ux-foundation.org, travis@....com,
	linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors


* Tejun Heo <tj@...nel.org> wrote:

> Rusty Russell wrote:
> > The generic versions Tejun posted are not softirq safe, so not
> > suitable for network counters. To figure out what semantics we
> > really want we need to we must audit the users; I'm sorry I haven't
> > finished that task (and won't until after the conference).
> 
> No, they're not.  They're preempt safe as mentioned in the comment and 
> is basically just generalization of the original x86 versions used by 
> x86_64 on SMP before pda and percpu areas were merged.  I agree that 
> it's something very close to local_t and it would be nice to see those 
> somehow unified (and I have patches which make use of local_t in my 
> queue waiting for dynamic percpu allocation).
> 
> Another question to ask is whether to keep using separate interfaces for 
> static and dynamic percpu variables or migrate to something which can 
> take both.

Also, there's over 400 PER_CPU variable definitions in the kernel, while 
only about 40 dynamic percpu allocation usage sites. (in that i included 
the percpu_counter bits used by networking)

So our percpu code usage is on the static side, by a large margin.

	Ingo
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