linux-kernel - Re: [PATCH v5 10/10] module: Rework module_addr

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Date:	Tue, 14 Apr 2015 15:00:41 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	rusty@...tcorp.com.au, mathieu.desnoyers@...icios.com,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, rostedt@...dmis.org, tglx@...utronix.de,
	laijs@...fujitsu.com, linux@...izon.com
Subject: Re: [PATCH v5 10/10] module: Rework module_addr_{min,max}


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Apr 13, 2015 at 06:56:36PM +0200, Ingo Molnar wrote:
> > > + * Bounds of module allocation, for speeding up __module_address.
> > > + * Protected by module_mutex.
> > > + */
> > > +static unsigned long module_addr_min = -1UL, module_addr_max = 0;
> > 
> > I suspect the same .data vs. .bss problem affects the #else branch as 
> > well?
> 
> Yes, but the linear walk already has a 'problem', other than the linear
> walk itself being one, the list_head isn't actually on the same line as
> the 'key' entries -- although I suppose I could fix that for the
> !CONFIG_MODULES_TREE_LOOKUP case.
> 
> > If so then it would make sense IMHO to put the structure definition 
> > into generic code so that both variants benefit from the shared 
> > cacheline?
> 
> Isn't this optimizing hopeless code? I mean, I can make the change; 
> something like the below. Although I suppose we should use 
> ____cacheline_aligned here and just take the false sharing.

Well, I think the point is to share more code and move the two 
variants closer to each other without hurting either side - not to 
optimize the slower side necessarily.

But I have no strong opinions either way!

Thanks,

	Ingo
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