linux-kernel - Re: [this_cpu_xx V7 0/8] Per cpu atomics in core allocators and cleanup

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Date:	Thu, 17 Dec 2009 15:25:58 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	Mel Gorman <mel@....ul.ie>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [this_cpu_xx V7 0/8] Per cpu atomics in core allocators and
	cleanup

* Christoph Lameter (cl@...ux-foundation.org) wrote:
> > However, I would need:
> >
> > this_cpu_cmpxchg(scalar, oldv, newv)
> >   (maps to x86 cmpxchg)
> >
> > this_cpu_add_return(scalar, value)
> >   (maps to x86 xadd)
> >
> > too. Is that a planned addition ?
> 
> It was not necessary. Its easy to add though.
> 
> > (while we are at it, we might as will add the xchg instruction,
> > althrough it has an implied LOCK prefix on x86).
> 
> Well yeah thats a thorny one. One could use the cmpxchg instead?

Yes, although maybe it would make sense to encapsulate it in a xchg
primitive anyway, in case some architecture has a better xchg than x86.
For instance, powerpc, with its linked load/store conditional, can skip
a comparison for xchg that's otherwise required for cmpxchg.

Some quick test on my Intel Xeon E5405:

local cmpxchg:  14 cycles
xchg:           18 cycles

So yes, indeed, the non-LOCK prefixed local cmpxchg seems a bit faster
than the xchg, given the latter has an implied LOCK prefix.

Code used for local cmpxchg:
                old = var;
                do {
                        ret = cmpxchg_local(&var, old, 4);
                        if (likely(ret == old))
                                break;
                        old = ret;
                } while (1);

Thanks,

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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