linux-kernel - Re: [patch 04/18] smp: Provide generic idle thread allocation

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Message-ID: <20120420162103.GA3065@merkur.ravnborg.org>
Date:	Fri, 20 Apr 2012 18:21:03 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Matt Turner <mattst88@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Mike Frysinger <vapier@...too.org>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Richard Kuo <rkuo@...eaurora.org>,
	Tony Luck <tony.luck@...el.com>,
	Hirokazu Takata <takata@...ux-m32r.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	David Howells <dhowells@...hat.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	"David S. Miller" <davem@...emloft.net>,
	Chris Metcalf <cmetcalf@...era.com>,
	Richard Weinberger <richard@....at>, x86@...nel.org
Subject: Re: [patch 04/18] smp: Provide generic idle thread allocation

On Fri, Apr 20, 2012 at 01:05:45PM -0000, Thomas Gleixner wrote:
> All SMP architectures have magic to fork the idle task and to store it
> for reusage when cpu hotplug is enabled. Provide a generic
> infrastructure for it.
> 
> Create/reinit the idle thread for the cpu which is brought up in the
> generic code and hand the thread pointer to the architecture code via
> __cpu_up().
> 
> Note, that fork_idle() is called via a workqueue, because this
> guarantees that the idle thread does not get a reference to a user
> space VM. This can happen when the boot process did not bring up all
> possible cpus and a later cpu_up() is initiated via the sysfs
> interface. In that case fork_idle() would be called in the context of
> the user space task and take a reference on the user space VM.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Matt Turner <mattst88@...il.com>
> Cc: Russell King <linux@....linux.org.uk>
> Cc: Mike Frysinger <vapier@...too.org>
> Cc: Jesper Nilsson <jesper.nilsson@...s.com>
> Cc: Richard Kuo <rkuo@...eaurora.org>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Hirokazu Takata <takata@...ux-m32r.org>
> Cc: Ralf Baechle <ralf@...ux-mips.org>
> Cc: David Howells <dhowells@...hat.com>
> Cc: "James E.J. Bottomley" <jejb@...isc-linux.org>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> Cc: Paul Mundt <lethal@...ux-sh.org>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Chris Metcalf <cmetcalf@...era.com>
> Cc: Richard Weinberger <richard@....at>
> Cc: x86@...nel.org
> ---
>  arch/Kconfig        |    3 +
>  kernel/cpu.c        |    2 -
>  kernel/sched/core.c |    2 +
>  kernel/smpboot.c    |   83 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/smpboot.h    |   10 ++++++
>  5 files changed, 99 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/arch/Kconfig
> ===================================================================
> --- linux-2.6.orig/arch/Kconfig
> +++ linux-2.6/arch/Kconfig
> @@ -145,6 +145,9 @@ config HAVE_DMA_ATTRS
>  config USE_GENERIC_SMP_HELPERS
>  	bool
>  
> +config GENERIC_SMP_IDLE_THREAD
> +       bool
> +

Symbols we select in arch Kconfig files are often named HAVE_*
And no matter the name - a comment preceeding the definition of the symbol
would be nice so it is possible to deduct the use of the symbol
in a few years time when this thread is long forgotten.

	Sam
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