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

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Date:	Mon, 20 Nov 2006 21:13:35 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Paul E. McKenney" <paulmck@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

On Mon, Nov 20 2006, Alan Stern wrote:
> On Mon, 20 Nov 2006, Jens Axboe wrote:
> 
> > On Mon, Nov 20 2006, Alan Stern wrote:
> > > Paul:
> > > 
> > > Here's my version of your patch from yesterday.  It's basically the same, 
> > > but I cleaned up the code in a few places and fixed a bug (the sign of idx 
> > > in srcu_read_unlock).  Also I changed the init routine back to void, since 
> > > it's no longer an error if the per-cpu allocation fails.
> > > 
> > > More importantly, I added a static initializer and included the fast-path 
> > > in synchronize_srcu.  It's protected by the new symbol 
> > > SMP__STORE_MB_LOAD_WORKS, which should be defined in arch-specific headers 
> > > for those architectures where the store-mb-load pattern is safe.
> > 
> > Must we introduce memory allocations in srcu_read_lock()? It makes it
> > much harder and nastier for me to use. I'd much prefer a failing
> > init_srcu(), seems like a much better API.
> 
> Paul agrees with you that allocation failures in init_srcu() should be 
> passed back to the caller, and I certainly don't mind doing so.
> 
> However we can't remove the memory allocation in srcu_read_lock().  That
> was the point which started this whole thread: the per-cpu allocation
> cannot be done statically, and some users of a static SRCU structure can't
> easily call init_srcu() early enough.
> 
> Once the allocation succeeds, the overhead in srcu_read_lock() is minimal.

It's not about the overhead, it's about a potentially problematic
allocation.

-- 
Jens Axboe

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