linux-kernel - Re: [PATCH] percpu: add optimized generic percpu accessors

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Message-Id: <200901291236.48189.rusty@rustcorp.com.au>
Date:	Thu, 29 Jan 2009 12:36:46 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tejun Heo <tj@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org, hpa@...or.com, brgerst@...il.com,
	ebiederm@...ssion.com, cl@...ux-foundation.org, travis@....com,
	linux-kernel@...r.kernel.org, steiner@....com, hugh@...itas.com,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors

On Wednesday 28 January 2009 21:26:34 Tejun Heo wrote:
> Hello,

Hi Tejun,

> Rusty Russell wrote:
> > If the stats are only manipulated in one context, than an atomic
> > requirement is overkill (and expensive on non-x86).
> 
> Yes, it is.  I was hoping it to be not more expensive on most archs.
> It isn't on x86 at the very least but I don't know much about other
> archs.

Hmm, you can garner this from the local_t stats which were flying around.
(see Re: local_add_return from me), or look in the preamble to
http://ozlabs.org/~rusty/kernel/rr-latest/misc:test-local_t.patch ).

Of course, if you want to be my hero, you could implement "soft" irq
disable for all archs, which would make this cheaper.

> > Other than the shouting, I liked Christoph's system:
> > - CPU_INC = always safe (eg. local_irq_save/per_cpu(i)++/local_irq_restore)
> > - _CPU_INC = not safe against interrupts (eg. get_cpu/per_cpu(i)++/put_cpu)
> > - __CPU_INC = not safe against anything (eg. per_cpu(i)++)
> > 
> > I prefer the name 'local' to the name 'cpu', but I'm not hugely fussed.
> 
> I like local better too but no biggies one way or the other.

Maybe kill local_t and take the name back.  I'll leave it to you...

> > Ah, I did not realize that you celebrated Australia day :)
> 
> Hey, didn't know Australia was founded on lunar New Year's day.
> Nice. :-)

That would have been cool, but no; first time in 76 years they matched tho.

Thanks,
Rusty.
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