Date: Tue, 20 Jan 2009 11:36:06 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Tejun Heo <tj@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
       Herbert Xu <herbert@gondor.apana.org.au>, akpm@linux-foundation.org,
       hpa@zytor.com, brgerst@gmail.com, ebiederm@xmission.com,
       cl@linux-foundation.org, travis@sgi.com, linux-kernel@vger.kernel.org,
       steiner@sgi.com, hugh@veritas.com,
       "David S. Miller" <davem@davemloft.net>, netdev@vger.kernel.org,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
Message-ID: <20090120103606.GA29346@elte.hu>
References: <20090115183942.GA6325@elte.hu> <200901170827.33729.rusty@rustcorp.com.au> <20090116220832.GB20653@elte.hu> <200901201328.24605.rusty@rustcorp.com.au> <49756E6E.2060409@kernel.org>
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* Tejun Heo <tj@kernel.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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