linux-kernel - Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

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Date:	Mon, 29 Feb 2016 14:08:34 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ben Maurer <bmaurer@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	linux-api <linux-api@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Dave Watson <davejwatson@...com>,
	rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Chris Lameter <cl@...ux.com>, Andi Kleen <andi@...stfloor.org>,
	Josh Triplett <josh@...htriplett.org>,
	Paul Turner <pjt@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Andrew Hunter <ahh@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

On Monday 29 February 2016 12:41:49 Mathieu Desnoyers wrote:
> ----- On Feb 29, 2016, at 5:39 AM, Arnd Bergmann arnd@...db.de wrote:

> > What's making things worse is that on some architectures, adding
> > __packed will force access by bytes rather than just reading
> > a 32-bit or 64-bit numbers directly, so it's slow and non-atomic.
> 
> Agreed that many architectures issue slower instructions when reading
> from packed structures, which is unwanted.
> 
> Could we require that each field be naturally aligned and require that
> they are placed so _no_ padding whatsoever should ever be added by the
> compiler ? If that's possible, then we could remove the packed.

Yes, I think that is a reasonable requirement.

	Arnd