linux-kernel - Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread

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Message-ID: <20160127180353.GB7514@cloud>
Date:	Wed, 27 Jan 2016 10:03:53 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Paul Turner <pjt@...gle.com>,
	Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
	rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of
 running thread

On Wed, Jan 27, 2016 at 05:36:48PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@...utronix.de 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.

If getcpu_cache doesn't exist, you'll get ENOSYS.  If getcpu_cache
returns 0, then you can assume the kernel will give you a valid CPU
number.

- Josh Triplett