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 08:39:58 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Lameter <cl@...ux-foundation.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

* Tejun Heo (tj@...nel.org) wrote:
> Hello,
> 
> On 12/16/2009 10:40 AM, Mathieu Desnoyers wrote:
> > Nope, there is the same number of sub-buffers for each per-cpu buffer.
> > I just want to see if supplementary indirections are allowed after
> > dereferencing the per-cpu pointer ?
> 
> Hmmm... you can store percpu pointer to a variable.  If there are the
> same number of commit_count for each cpu, they can be allocated using
> percpu allocator and their pointers can be stored, offset and
> dereferenced.  Would that be enough?

Yes, I think I could allocate, from the channel structure perspective:

- A percpu pointer to the per-cpu buffer structures
- A percpu pointer to the per-cpu commit counters.

This should fix my problem. The main change here is that the pointer to
the commit counters would not be located in the per-cpu buffer
structures anymore.

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 ?

(while we are at it, we might as will add the xchg instruction,
althrough it has an implied LOCK prefix on x86).

Thanks,

Mathieu

> 
> Thanks.
> 
> -- 
> tejun

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