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

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Date:	Fri, 17 Nov 2006 23:33:45 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	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>,
	Andrew Morton <akpm@...l.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, 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?

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

> For the moment, I cheaped out and used a mutex_trylock.  If this can block,
> I will need to add a separate spinlock to guard per_cpu_ref allocation.

I haven't looked at your revised patch yet...  But it's important to keep 
things as simple as possible.

> Hmmm...  How to test this?  Time for the wrapper around alloc_percpu()
> that randomly fails, I guess.  ;-)

Do you really want things to continue in a highly degraded mode when 
percpu allocation fails?  Maybe it would be better just to pass the 
failure back to the caller.

Alan Stern

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