From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number
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Date: Wed, 27 Jan 2016 17:36:48 +0000 (UTC)
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To: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Cc: Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>, Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>, Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>, Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>, rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>, Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
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----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:

> On Wed, 27 Jan 2016, Josh Triplett wrote:
>> With the dynamic allocation removed, this seems sensible to me.  One
>> minor nit: s/int32_t/uint32_t/g, since a location intended to hold a CPU
>> number should never need to hold a negative number.
> 
> You try to block the future of computing: https://lwn.net/Articles/638673/

Besides impossible architectures, there is actually a use-case for
signedness here. It makes it possible to initialize the cpu number
cache to a negative value, e.g. -1, in userspace. Then, a check for
value < 0 can be used to figure out cases where the getcpu_cache
system call is not implemented, and where a fallback (vdso or getcpu
syscall) needs to be used.

This is why I have chosen a signed type for the cpu cache so far.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com