linux-kernel - Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall

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Date:	Fri, 17 Nov 2006 20:51:03 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...l.org>,
	Thomas Gleixner <tglx@...esys.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>,
	David Miller <davem@...emloft.net>,
	Arjan van de Ven <arjan@...radead.org>,
	Andi Kleen <ak@...e.de>, <manfred@...orfullife.com>,
	<oleg@...sign.ru>
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

On Fri, 17 Nov 2006 23:33:45 -0500 (EST)
Alan Stern <stern@...land.harvard.edu> wrote:

> On Fri, 17 Nov 2006, Paul E. McKenney wrote:
> 
> > > Perhaps a better approach to the initialization problem would be to assume 
> > > that either:
> > > 
> > >     1.  The srcu_struct will be initialized before it is used, or
> > > 
> > >     2.  When it is used before initialization, the system is running
> > > 	only one thread.
> > 
> > Are these assumptions valid?  If so, they would indeed simplify things
> > a bit.
> 
> I don't know.  Maybe Andrew can tell us -- is it true that the kernel runs 
> only one thread up through the time the core_initcalls are finished?

I don't see why - a core_initcall could go off and do the
multithreaded-pci-probing thing, or it could call kernel_thread() or
anything.  I doubt if any core_initcall functions _do_ do that, but there
are a lot of them.

> If not, can we create another initcall level that is guaranteed to run 
> before any threads are spawned?

It's a simple and cheap matter to create a precore_initcall() - one would
need to document it carefully to be able to preserve whatever guarantees it
needs.

However by the time the initcalls get run, various thing are already
happening: SMP is up, the keventd threads are running, the CPU scheduler
migration threads are running, ksoftirqd, softlockup-detector, etc. 
keventd is the problematic one.

So I guess you'd need a new linker section and a call from
do_pre_smp_initcalls() or thereabouts.
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