netdev - Re: Poll about irqsafe_cpu

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Date:	Thu, 17 Mar 2011 14:21:33 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Poll about irqsafe_cpu_add and others

On Thu, 17 Mar 2011, Eric Dumazet wrote:

> > > I wonder why we dont use :
> > >
> > > 	addl	$0x2,%fs:xt_u64
> > > 	addcl	$0x0,%fs:xt_u64+4
> >
> > The compiler is fed the following
> >
> > 	*__this_cpu_ptr(xt_u64) += 2
> >
> > __this_cpu_ptr makes it:
> >
> > 	*(xt_u64 + __my_cpu_offset) += 2
> >
> > So the compiler calculates the address first and then increments it.
> >
> > The compiler could optimize this I think. Wonder why that does not happen.
>
> Compiler is really forced to compute addr, thats why.
>
> Hmm, we should not fallback to generic ops I think, but tweak
>
> percpu_add_op() {

percpu_add_op() is not used. This is a 64 bit operation on a 32 bit
machine thus we fall back to this_cpu_generic_to_op()

#define __this_cpu_generic_to_op(pcp, val, op)                          \
do {                                                                    \
        *__this_cpu_ptr(&(pcp)) op val;                                 \
} while (0)


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