linux-kernel - Re: [thiscpuops upgrade 08/10] percpu: generic this_cpu_cmpxchg() and this_cpu_cmpxchg

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Date:	Fri, 26 Nov 2010 18:01:27 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Lameter <cl@...ux.com>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [thiscpuops upgrade 08/10] percpu: generic this_cpu_cmpxchg()
 and this_cpu_cmpxchg_double support

Le vendredi 26 novembre 2010 à 17:58 +0100, Tejun Heo a écrit :
> On 11/26/2010 05:56 PM, Eric Dumazet wrote:
> > Le vendredi 26 novembre 2010 à 17:51 +0100, Tejun Heo a écrit :
> >> On 11/24/2010 12:51 AM, Christoph Lameter wrote:
> >>> +/*
> >>> + * cmpxchg_double replaces two adjacent scalars at once. The first parameter
> >>> + * passed is a percpu pointer, not a scalar like the other this_cpu
> >>> + * operations. This is so because the function operates on two scalars
> >>> + * (must be of same size). A truth value is returned to indicate success or
> >>> + * failure (since a double register result is difficult to handle).
> >>> + * There is very limited hardware support for these operations. So only certain
> >>> + * sizes may work.
> >>> + */
> >>> +#define __this_cpu_generic_cmpxchg_double(pcp, oval1, oval2, nval1, nval2)	\
> >>
> >> Ah... this is scary.  If it proves to be useful enough, sure, but
> >> otherwise I'd like to avoid it.
> > 
> > This is mandatory for several lockless algos, dont be afraid, its a
> > single instruction ;)
> 
> Thanks for holding my hands. :-)
> 
> What I'm afraid of is generic code switching to use it in very hot
> paths when a lot of archs can't actually do it leading to performance
> regressions.  Is this something which is available on most archs?
> 

I would say cmpxchg() has the same problem, some arches dont have a
native instruction, yet we use it in some paths.



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