netdev - Re: [PATCH] percpu: add optimized generic percpu accessors

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Date:	Sat, 31 Jan 2009 15:11:14 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
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

Hello, Rusty.

Rusty Russell wrote:
>> 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 ).

Ah... Great.

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

I suppose you mean deferred execution of interrupt handlers for quick
atomicities.  Yeah, that would be nice for things like this.

>>> 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.

It was a joke.  :-)

Thanks.

-- 
tejun
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